Why use a fieldbus?
Using fieldbus systems for automation tasks gives a lot of important advantages compared to the usual point-to-point coupled systems:
- It saves a lot of cables and cabling and thereby a lot of money.
- Any errors are immediately detected. A traditional 4-20 mA signal may give wrong readings for years if water
penetrates the cable or a junction box, and a traditional switch signal may not work on demand if it for example is shorted to ground.
- It is possible with a far better diagnostics with warning and error telegrams.
- It is fast and easy to add new actuators and sensors and calibrate them.
- Because there is a standard communication protocol, it is easy to connect even very complex equipment from different vendors. Some
investigations has pointed out that this reason is even more important than saving money!
- If the fieldbus has for example a WiFi connection, it is possible to use cheap sensors without local display. You simply type
in the number of the sensor in an app on your mobile phone and can then read the value in the display. In this way, it is also easy to read hard-to-reach sensors and
for example compare measuring values from parallel lines.
The disadvantages of using fieldbus systems has previously been:
- Reduced speed.
- Reduced reliability.
- Bad failure tolerance.
- Increased sensibility to noise due to very low signal energy and big signal bandwidth.
- Switch problems due to low voltages. Traditional mechanical switches need 48 V for a reliable operation in industrial environments, which means
that a -24 V load voltage is needed in case of standard 24 V systems! Almost no control systems including fieldbus systems have that except for the
Innovatic LC3000 module.
Although no fieldbus can completely match a hard-wired solution when it comes to these points, Max-i tries to cope with all these problems:
- It is fast enough for most applications.
- It is as reliable as most PLC I/O channels.
- It may still work even if the bus is cut into more pieces.
- The signaling level is close to 12 V, which is much higher than any other fieldbus system and gives a better signal/noise ratio than
any hard-wired 4-20 mA or 0-10 V signaling.
- The single chip interface makes it possible to build the interface into the sensors and in this way use magnetic activated switches or
other switches without low-voltage problems.
Max-i enables the lowest total automation cost ever seen! This is one of the most important properties since many customers primary
focus on price. Therefore, Max-i has been designed to reduce the price of all parts of the automation process - not just the price of the
- With Max-i, it is possible with a complete bus interface in one single simple and cheap IC (Integrated
Circuit), which gets its supply voltage directly from the bus and uses an internal RC oscillator (with ±8%
accuracy). There is no need for crystals, resonators or any other external components except for a small decoupling
capacitor - not even components for overload or transient protection as the connection between the IC and the actuator
or sensor is not taken out of the unit. The one-component solution may even save the usual costs for an electronic
production with printed circuit board, component mounting, soldering, testing etc. if the IC is mounted in a socket. With Max-i, a complete bus
interface may be cheaper than a normal general purpose I/O channel! The single chip interface makes it for the first
time both technical and economical possible with a direct bus interface to all devices - even the
simplest price sensitive ones like motor starters, valves, limit switches, push-buttons, lamps and sensors.
- With Max-i, it is possible to control an entire process plant by means of one or more cheap Soft-PLC/SCADA (Software based
Programmable Logic Controller / Supervisory, Control and Data Acquisition) systems, a bus power supply and a bus cable. This enables
very big savings in the form of discrete PLC's, distributed I/O systems, mounting boxes, clamp rows, adapters, cabling etc.
- Max-i saves a lot of money for cabling and mounting compared to other fieldbus systems.
- Max-i may use cheap unshielded standard installation cables. There is no need for expensive and troublesome shielded cables or
for twisted pair cables! In case of a 4-wire line, the cable should be "green", that is, with a polyolefin based conductor insulation such as PE,
FRPE, PP etc. (Er < 2.68). For use in houses, a standard 3-wire installation cable may be used - even with PVC insulation, and for cars and
aircrafts where there is a common chassis, it is enough with a simple 2-wire line or a coaxial cable.
- In many cases, it is only necessary with one cable, as it is possible to connect up to approximately 1,000 devices depending on the cable
length, conductor cross-section and the used semiconductor technology. The cable may be up to approximately 8 km long or 16 km in a closed ring -
- It is not necessary with an extra cable for power supply as it is possible to
transfer an almost unlimited amount of power over the bus (depends only on the conductor
cross section). There is therefore no need for a galvanic separation (between bus and
power supply), which is a condition for making a complete bus interface in one IC.
- Because of an outstanding noise immunity there is no restrictions on where and how the bus cable may be drawn. In this
way, it is possible to use existing cable ducts. Other fieldbus systems usually requires that the bus cable shall be drawn a
distance from power cables, cables from frequency converters etc. or shall even cross such cables in a right angle. This may make it
necessary to mount new cable ducts.
- If an electronic motor starter and a safety fuse is integrated with the motor, even bigger savings are possible as the power cable
may then also be drawn from motor to motor. This is the future way of doing automation! Note that because of the outstanding noise immunity
of Max-i, the two cables (fieldbus and power) may be drawn in parallel in the same cable duct.
- It is allowed to use drop cables with a length up to 3 m or even longer for Max-i controllers, which can reduce the rise time.
- Due to a very simple possibility for making a self-healing ring (described below) it may be possible to make failure tolerant networks
much cheaper than with other bus systems, which may require two cables and double interface to obtain the same wanted safety.
Max-i uses a nominal bus voltage of 20 Vdc (14.4 - 21.6 V). This is low enough for modern semiconductor technology, but
high enough to enable a fairly big supply power. Besides, it is very safe as the arc risk is very limited as described in details in Annex E of the specification.
At 18-21 V, short arcs up to approximately 0.5 mm may occur, but at higher voltages like 24-28 V, arcs up to 10 mm are possible if sufficient current is available
so it may be very difficult to break a DC supply in case of a short circuit.
The industrial standard today is 24 V and the standard for aerospace is 28 V, but they are based on the requirements for mechanical contacts,
electromagnetic relays (motor starters) and the charging voltage of 24 V batteries (aerospace). In the future, it is expected that mechanical sensors will be replaced
with much more reliable integrated magnetic sensors like hall elements build into the fieldbus interface, and the old starter
contactors will be replaced with electronic soft starters or frequency converters where 20 Vdc is the ideal supply
voltage for the low-voltage part. Besides, since Max-i uses a 4-wire balanced cable in industrial and aerospace environment, it is anyway not possible just to connect
it to an existing unbalanced power supply, so it is not necessary to use the same voltage level.
Because Max-i may use standard installation cables, it is practical possible with an output power up to approximately 800 W per segment/power-supply on
4 mm2 cables in a ring structure. This makes Max-i to a true actuator bus with power enough for driving several motor starters, pneumatic and hydraulic valves,
lamps etc. Most other fieldbus systems, which supplies the devices through the bus cable, are really only sensor busses due to their very limited current capability of
typical below 2 A. Such a low current may make it hard to make a direct bus interface to the various devices because it may limit the number of
devices on each bus to a level, which is not appropriate. In a typical process plant there may be over 100 sensors and over 50 big actuators and valves
per bus, which may require a total of up to 20 A, which no other fieldbus than Max-i is able to handle! With most other fieldbus systems, the number of
busses necessary to supply the required current may be so high that the use of direct bus interface is not realistic!
The typical current requirement at 20 Vdc for a modern low energy actuator will be:
|Three phase 400 V contactor
However, the power consumption of older (existing) actuators may be
Max-i may be regarded as a 20 V supply with communication. This makes Max-i the ideal choice for low voltage
supply in low energy houses of the future. Today, most houses only have a 115 Vac or 230 Vac supply, but many equipments could
benefit from a 20 V supply instead of having each a clumsy converter with often very low efficiency for example for equipment like:
- Intelligent lighting control including advanced LED lighting, professional stage light and architectural lighting.
- Window openers.
- Intelligent sun shielding.
- Electronic lock systems - even in hotels with hundreds of doors.
- Coupled door bells with door indication.
- Power supply for all kinds of computer peripherals and telecommunication products including scanners, monitors, modems, IP telephones and wireless phones.
- All kind of alarms including burglary alarms and baby alarms and coupled smoke detectors with battery backup, which is a demand in all new
buildings in Denmark.
- All kind of battery chargers for mobile phones, tablets, labtop computers, cameras, MP3 players, toys etc. including all levels of
USB Power Delivery and Quick Charge.
- Aquarium pumps, water art, small fountains etc.
- All kind of toys including LEGO, doll's houses, electrical trains and race tracks etc. - even with communication. In this way, 230 Vac outlets
may be switched off in kids rooms to prevent shock hazard.
- Weather stations.
- Heat recovery equipment with low energy fans partly driven by solar cells.
In the green house of the future, 115 Vac or 230 Vac outlets are mounted in all outer walls and in the ceiling, and Max-i outlets
with intelligent lighting control are used in all inner walls.
Calculations done by the Danish engineering company Rambøll indicates that a low voltage DC network partly driven by
solar cells can save Danish households in the order of 1 billion Danish kroner per year corresponding to approximately $150,000,000.
Max-i is also very useable as a power supply network for mobile phones and laptop computers in for example trains and busses. The communication makes
it possible to transfer low to medium speed information like information about route, next stations, expected arrival etc. to over 1000 units on
Number of Devices
The maximum number of devices depends on the used semiconductor technology (leakage currents and capacitance), the bus length and the conductor
cross-section. In practice, it possible with over 1000 devices on one bus. Together with the very high amount of power transfer, this
makes it possible to keep the number of bus lines at a realistic and appropriate low level.
With Max-i, it is usually not necessary with receptacle busses, hubs or routers.
By means of a brand-new equipment identification system - PNS (Plant Numbering System - see Plant Numbering) it is
very easy to manage the high number of devices and signals. Unlike all other fieldbus systems (except perhaps for CAN-B), Max-i has been designed to
use addressing directly by means of machine numbers instead of the usual special bus address (typically 0-63, 0-127 or 0-255) or a long
unique serial number (usually 48-64 bit). For example, HX361T2 means Heat Exchanger 361 Temperature 2. The machine number is known from the drawing and
is therefore easy to type in by means of for example an app on a mobile phone, and if this is done during the mounting of the device, there is no risk for any confusions.
With direct addressing on machine numbers, there is no need for the usual huge cross-reference tables, and it saves the trouble with managing bus
addresses and recording serial numbers during commissioning and service (device replacement). It is also a great advantage in the daily work where it
makes it possible to remote control the equipment by means of a simple Web- or SMS-interface to a mobile phone. If you stand in the middle of nowhere and
receives an error message from the plant, it is much easier if the error identifies itself as HX361A3 (alarm 3) than for example point 127!
The machine numbers use 31 bits, but for very time critical data, PNS also defines a 12-bit quick number format, which is compatible with the long
machine numbers and may be used on the same bus. The 31/12 bit addresses is compatible with the CAN A and B 11/29 bit addresses so that existing CAN
identifiers and numbering systems may be used on Max-i. In fact, it is possible to run for example a DeviceNet or CANOpen protocol on the quick-numbers
simultaneously with PNS (machine numbering) on the same bus - although there is no reason to do so. It will only add a complicated software overhead,
without gaining anything compared to what Max-i offers as standard.
High efficiency is the keyword for a good fieldbus system. Just increasing the speed for getting more data through, as it is common
practice with many of today's bus systems, leads to a critical bus timing and a bad signal to noise ratio, because the energy in each communication
pulse is reduced. Remember that energy is voltage multiplied by current multiplied by time. Each time the baud rate is doubled, the energy in each pulse
is reduced to the half, but the bigger necessary bandwidth means that the noise energy is approximately doubled so the signal to noise ratio
is reduced approximately 4 times! Therefore, Max-i has been designed for efficiency rather than speed! What really counts is the number of telegrams
per second. A high communication speed is just a means - not the aim!
- Max-i is a multi-master bus based on the CSMA-CD+CR principle (Carrier Sense Multiple Access with Collision Detection and Collision
Resolution). This gives a much better utilization of the bandwidth than any TT (Time Triggered), TDMA (Time Division Multiple Access), Token-passing,
Token-Slot or master/slave networks. There are no dedicated master or slave devices, no channel generators etc. and only one type of device! Any
number of bus nodes can request the bus simultaneously and any node may initiate a transmission. This enables very fast response times even
at a low baud rate and with many devices. The bus arbitration is non-destructive so that no bandwidth is lost in case of a collision.
The multi-master concept also makes it possible to connect for example a separate web interface, a debugger or a bus tester. This is extremely
practical during commissioning or in those situations where a vendor wants access to a device for for example remote service, but the plant has no
- Unlike for example TT, TDMA, Token-passing and Summation-frame networks it is not necessary to reconfigure the bus when devices are added or
removed or if a device fails. The only exception is when Max-i is used to transfer data to many devices simultaneously in the same telegram.
- Max-i uses synchronous communication so that no time is wasted on start- and stop bits.
- Max-i has a unique polling method where the first and last part of a telegram is transmitted by two or more devices so that it does not take
more time to poll a value than to send it event driven! This increases the
efficiency a lot compared to most other bus systems - especially with many safety values or analog measurement values, which are typically polled.
- Max-i uses the very efficient producer/consumer model known from CAN. With this model, it is each value, which has an address - not
the bus node! This address is called the identifier. Most telegrams like input data from the process may be regarded as broadcast
information no matter if they are initiated by an event or polled! Many devices may therefore receive the same message and
benefit from data requested by other devices. This is very useful in for example redundant SCADA systems where the method efficiently reduces the
necessary number of telegrams and guarantees data consistency - even without a common database - so that all devices show exactly the same
values. It is also very efficient for synchronizing for example clock, program execution, set points etc. in more devices, and it is very efficient
for information systems where the same information shall be shown on many displays. As every single value - even Boolean values - has its own
identifier, there is no need for AND and OR functions etc.
The producer/consumer model has the further advantage that there is no need for address stacking and address conversion in case of
gateways between different bus systems. This reduces the length of the telegrams and makes the gateway function simple and fully transparent,
so that a gateway may be added or removed at any time without any changes in the bus nodes.
- The Max-i telegrams are very compact with a very little overhead, and it uses separate error and warning telegrams so that it
is not necessary to include diagnostic information in all telegrams. With the short identifier, it is only necessary with 7 bytes to transmit a
fixed point 24-bit process value in SI-units, and Boolean 2-bit values may be transmitted in only 5 bytes.
- Max-i is able to group devices together in up to 255 different groups, which may be switched off and back on by means of a common telegram.
This is very useful in lighting control where it is very desirable to be able to switch groups of lamps and devices with potential fire hazard off
when you leave the house or go to sleep. It is also very useful for energy management where it makes it very easy to switch for example electrical
heating and wash machines off in periods with low "green" supply or very high power consumption.
- Max-i is able to send 16-bit and/or 32-bit data to up to 256 devices simultaneously in one standard telegram and/or one group telegram. This increases
the efficiency a lot for example for stage light and positioning systems, where the common telegram also guarantees 100 % synchronization.
- To make it possible to transfer as many telegrams per second as the network length allows, Max-i has 12 speeds. The table below shows
the relationship between the bus length, the minimum cable cross section, the number of telegrams per second with 12-bit and 31-bit identifiers, the
Max-i baud-rate and the communication speed when a Max-i controller is connected to a serial
port for example by means of an RS-232 or RS-485 interface.
The minimum cable cross-section implies that the maximum distance to a power supply is ½ the maximum cable length and that the bus
runs at the maximum speed according to the cable length. If a speed reduction to the half or one quarter can be accepted, the cable cross-section may
also be reduced correspondingly to the half or one quarter. This is shown with the gray fields in the table. Because the maximum practical cable
cross-section is 6 mm6, a distance of 8 km is in practice only possible at a reduced speed. Please see the specification for further details.
The 18-bit telegrams are used for transmission of process values in (modified) SI-units (meter, kg, °C, Bar, Volt, Ampere etc.) with an
accuracy of up to 17 bits plus sign. In practice, this is all process values except perhaps for weight signals. Only 18 bits are needed,
because the (fixed) scale factor (exponent) (see PNS data type FIX) is embedded in the Max-i fieldbus protocol. This
improves the efficiency and saves two bytes compared to the IEEE 754 single-precision floating-point format.
The number of telegrams per second for a given network length is quite outstanding compared to many other fieldbus systems, and in many cases
it is more than most SCADA systems are able to handle - especially with more bus lines! A typical Windows or Linux based SCADA system can
only handle approximately 300 telegrams per second! Therefore, Max-i has two group of signals - high speed local signals and low to medium speed
signals intended for SCADA systems. Unlike CAN, it is possible for a SCADA system to pick out samples from a high-speed signal without being
Even though Max-i is a multi-master bus based on a simple RC oscillator and has a 22-bit CRC (Cyclic Redundancy Check) and separate
error, warning and configuration telegrams, it is approximately as efficient as ASI, which is a crystal controlled single-master bus with
only a simple one-bit parity check and no possibility for error, warning and configuration telegrams. On a 100 m bus, ASI is able to
address 31 devices with 4-bit data (124 bits) every 5 mS. If a corresponding 6-bit identifier (64 devices with 2-bit data = 128 bit) and the same
cable length were used on Max-i, it would be able to transfer the same number of bits every 6.4 mS. This is 27% slower than ASI, but
because Max-i is an event driven multi-master bus, it would be much faster in practice with a response time approximately 25 times less
than ASI (0.1 mS compared to an average of 2.5 mS)!
Max-i-mum Bus Length
The maximum bus length without repeaters (8 km) is very close to the maximum length of other copper based fieldbus system with
repeaters. For example, the absolute maximum length of a copper based Profibus with repeaters is 10 km and the maximum length of Interbus is 13 km, but
Interbus uses all devices as repeaters.
Max-i is the only fieldbus, which is 100% compatible with all kind of industrial environments!
- It is allowed to draw the bus cable in parallel with any other cable so that it is not necessary with new cable ducts.
- For industrial applications, Max-i uses a fully floating transmission line, that is,
no part of the bus has a direct ground (earth) connection. This is very important in industrial environment where the bus cable is
probably unable to handle a short circuit current. However, it is also very important even during normal operation! Many process plants use
power networks with a directly grounded neutral (TN-C system), where a common neutral and safety conductor (PEN) is used instead of a separate
protective grounding. If there is any difference in the loading of the 3 phase's in such a system the PEN conductor will draw current. During
worst case conditions, where the currents are not sinusoidal, they may add together so that the PEN conductor theoretically seen may draw up
to 3 times the current in a phase conductor - 2 times is quite common in practice. The current in the PEN conductor will create an AC voltage
between different chassis parts and a current may then flow between these if they are connected. Depending on the impedance level, the
current levels may be quite high. More than 20 Aac has for example been measured in a water pipe and 70 Aac in a handrail! It is obvious
that it is not a good idea to connect for example a cable screen between different chassis parts of such a process plant and in this way establish a
low impedance current path, which may disturb the communication and may heat or even melt the cable. Nevertheless, this is common practice with
almost all shielded fieldbus systems!
Note, that with a fully floating bus nothing is gained by using a shielded cable! This saves indeed a lot of money and problems!
To avoid electrostatic build-up, to avoid breakdown of the galvanic separations in case of a powerful transient and to be able to
detect a failure current, Max-i is connected to ground through a network consisting of resistors and a heavy duty spark gap (gas discharge
tube). The voltage on the network is supervised and an alarm is activated if the leakage current exceeds a given limit. Due to the
floating cable, most transients will not cause a high current in the cable and will therefore not stress the protection components.
Max-i has an unsurpassed noise reduction and a signal/noise (S/N)
ratio, which for example is approximately 150 times better (at 83.3 kBaud) than IEC 61158-2 based systems and
approximately 20 times better than CAN (at the same line length). There are more reasons for this:
- Max-i uses transmitter pulses with a very high energy of approximately 33 µJ at 111 kBaud on a 1 km closed ring. On a
corresponding 500 m trunk line (closed ring not possible), CAN has a maximum pulse energy of 1.6 µJ at 125 kbit/s.
In spite of the big transmitter power Max-i has a low radiated emission (EME) because of a very efficient bit
coding. The highest fundamental frequency is only 27.8 kHz at 55.6 kBaud so that the EME drops with 6dB/octave from this frequency.
- The receiver has a 1 V hysteresis (at 20 V) and an EMI (Electro Magnetic Interference) filter with approximately 1/2 pulse width like a UART.
- Max-i does not use any termination resistors! This creates many reflections, but Max-i utilizes these
to improve the S/N ratio.
- Max-i does not rectify noise up to approximately 7 Vpp, so noise usually does not generate any bias distortion.
- Unlike for example CAN based systems, the data coding is fully symmetrical and has no bias-distortion and no need for equalizers or preemphasis.
Bias distortion occur if the charge and discharge energy of the line is not the same. Due to the skin effect, a transmission line will always
behave like a low pass filter and therefore is able to be charged - even if it is terminated properly. This effect may be further strengthened by
capacitive loading, drop cables or different cable types. The drive impedance of a dominant CAN-bit is only a few ohm, but the impedance of a
recessive bit is equal to the line impedance, that is, 60 ohm for high-speed CAN and as high as 4.4 kohm for low-speed CAN. This create a difference
in the rise and fall time, and the line will have a tendency of being charged against the dominant state. This tendency is further strengthened by the
NRZ (Not Return to Zero) coding of CAN, because NRZ contains DC. A row of dominant 0-bits may charge the line so much that it is impossible to detect
a following recessive 1-bit.
- Unlike for example CAN based systems, the bus timing is uncritical even at maximum bus length and maximum clock inaccuracy! The average margin is
1/2 pulse width like a UART.
The CAN-bit is divided into a short synchronization segment, a propagation segment and two equal phase segments. The sample point is located
between the two phase segments. Because the length of the propagation segment shall be equal to two times the maximum delay from one device to another,
the length of the phase segments must be reduced when the line length is increased. This makes CAN very
critical at long transmission lines. At for example 125 kbit/s and a 500 m cable, the bit length is 8 µs, but the propagation segment must
be at least 5 µs with PE or PP insulated cables, so there is only 1.5 µs left for each phase segment. This makes
the timing approximately 3 times more critical than with short cables! In case of propagation delay and more devices, which tries to access the bus,
the width of the dominant bit may be increased with up to two times the propagation delay and the width of the recessive bit reduced correspondingly.
If there is also some bias distortion, the signal integrity may be completely destroyed as the recessive bit may be almost wiped out as shown below.
CAN is really not designed for propagation delay!
To make the situation even worth, the timing depends on whether the device is galvanic separated/insulated or not, and the sample point must be
programmed into all devices depending on the line length. If this is not done properly, CAN will not be reliable. In Max-i, there is never any
critical timing and nothing to program.
The excellent noise immunity of Max-i is illustrated below. The oscilloscope picture shows communication even under very heavy noise
conditions with a differential mode noise as high as the communication pulses and approximately the same pulse width. On a balanced line, it requires
an extreme noise level or a very heavy unbalance to generate noise levels that size.
For several reasons Max-i is one of the most reliable fieldbus systems.
- The unsurpassed S/N ratio makes Max-i to a zero-error system, that is, a system, which under normal circumstances should have an
error rate of zero. Other fieldbus systems typically have average failure rates of approximately 10-7, however, there is a
tremendous difference between zero errors and just a single error in time. If one error can occur so can 2, 3, 10 or even hundreds or
thousands. The error probability (P) depends heavily on the S/N ratio. It is typically of order e-(S/N), where the noise power N is
proportional to the baud-rate. This is the reason why Max-i focuses on efficiency rather than speed! If the S/N ratio is changed a factor k
the new error probability may be calculated as:
Pnew = e-k(S/N) = [e-(S/N)]k = Poldk
If for example the error probability is 0.01 and the S/N ratio is increased 3 times (k=3) then the new error probability will be approximately
0.013 = 0.000001. In the same way, the difference between 1 and 3000 errors in a system with an average error rate of
10-7 is only a factor 2 increase in noise level or a factor 1.4 increase in transmission speed!
- The very short telegrams increase the reliability because it is more likely that an error strikes a long telegram than a short one.
- The simple single chip interface to the various devices enables a very high reliability because no crystals, optocouplers or other low
reliability components are needed. If the IC is mounted in a socket, no soldering process is needed and there may therefore be
no solder joints to fail - a very common error source, and the components are not thermally stressed during the mounting.
- With the high number of devices on one bus and the absence of termination resistors, it is not necessary with routers etc. This
also increases the reliability. Other bus systems like Ethernet, FlexRay and systems based on light-guides uses point-to-point
connections from each device to a router so that the telegrams may have to pass several routers and with that a lot of electronics on its way.
- The high transmitter power keeps the communication well free of the low-signal range below approximately 5 V where a
reliable function of connectors etc. cannot be expected. This is a big problem with the majority of all other fieldbus systems like for example Ethernet,
all CAN and RS-485 based systems and systems based on LVDS (Low Voltage Differential Signaling).
With Max-i, it is even possible to obtain a higher reliability than a normal PLC (Programmable Logic Controller)! From a reliability
point of view, it does not matter if the bus, which connects the I/O's (Input/Output) to the CPU (Central Processing Unit), is a short fast
parallel bus internal in a PLC or a long relatively slow serial fieldbus. A typical PLC with a 16-bit internal bus
and 16 I/O channels per card uses statistically approximately 1-2 bus transceivers per channel plus one optocoupler. Max-i uses of course only
one bus transceiver per channel and saves the optocoupler. In this way, it is possible to obtain at least the same per-channel reliability as with a
PLC. Almost no other fieldbus system offers this degree of reliability.
The absence of termination resistors makes it possible to make a fault tolerant self-healing ring. This may be done in two ways:
In both cases, a very high function safety (not just failure safety) is obtained, which by far exceeds all bus systems, which
depends on termination resistors, like for example all CAN, RS-485 or LVDS based systems.
With the producer/consumer model and the direct bus interface to the various devices, it is very easy to make redundant systems because any
number of SCADA systems may be connected to the same bus and may utilize the same telegrams and I/O devices. A further advantage is that such a
system does not depend on common bus couplers as with distributed I/O systems or depend on a central database.
It is possible to change the input values temporary for example to compensate for sensor errors or to simulate the presence of material. This is
extremely useful during commissioning and can save a lot of time and money.
With the long 31-bit PNS identifier the probability that for example an error on a drawing or a typing error creates a new legal identifier is
extremely small as less than 1ppm (parts per million) of the possible identifiers are utilized! With traditional bus systems (or quick numbers),
the probability may be close to 100%!
A very efficient priority system with 4000 levels (12 bits) ensures that the most important telegrams comes first, and a unique "babbling-idiot"
protection ensures that no device or devices are able to saturate the bus. All telegrams always come through. Max-i is
therefore fully deterministic unlike for example CAN and Ethernet! The maximum response time is as short and predictable as for example
TT, TDMA, Token-passing and master/slave networks, but the average response time may be several hundred times faster
depending on the number of devices! The "babbling-idiot" protection has the further advantage that it is usually not necessary to worry about
priority levels. Unlike CAN, the various identifiers may therefore be chosen freely, which for example is a great advantage if PNS or another numbering system
To ensure that no device or SCADA system is using/showing obsolete information, Max-i has a standardized time-out system, which makes
the received values invalid after a given amount of time. This also makes it unnecessary to poll all devices to check if they are alive. A
hardware watch-dog timer on each channel may for example be used for setting the output low if the output has not been updated within a given amount
For safety systems in accordance with AK6 of DIN V 19250, category 4 of EN 954-1 and SIL 3 (Safety Integrity Level 3) of IEC 61508, the total system
error probability shall be less than 10-7 per hour (and cannot be claimed to be better than 10-8). To guarantee this, Max-i uses a
very special, but very simple 22-bit CRC check invented by Innovatic and developed in collaboration with the Technical University of Denmark - DTU. This
alone reduces the test time to less than 11 hours. If no errors are detected during this time, the demand is fulfilled with the required statistically
confidence level of 99%. However, Max-i also has a 7-bit Hamming check on the identifier, which further improves the probability for error detection.
Because a safety system trips no matter which error occurs, this means that if the equipment runs without very frequent error trips, the demand is
automatically fulfilled. If the long 31-bit identifier is used, it is possible to transmit 1 data byte and therefore all Boolean values with hamming
distance 8, that is, all errors up to 7 bits are detected with 100% probability. The hamming distance is 6 up to 25 data bytes and 4 up to 262143 data
bytes. If the short identifier is used, it is of course possible to transmit two more bytes with each hamming distance. With minor limitations, it is
even possible to transmit an infinite long telegram with hamming distance 4, 6 or 8! Unlike many other industrial fieldbus systems, it is therefore easy
to download programs and big data files.
For these kind of safety systems it is required with two-channel redundant and diverse hardware and data processing. Due to the direct bus interface,
the multi-master concept, the producer/consumer model, the fast polling system and the standardized time-out system it is possible to obtain this
without a special protocol or special hardware - except for one or more standard safety/emergency-stop relays.
To fulfill the requirement for redundancy, a safety switch or device must be coupled to the Max-i controller by means of a two-channel
interface. Max-i always uses 2-bit Boolean values so the Max-i interface circuit always has two input channels.
Each of the two channels has a fully redundant signal processing. Because of the hardwired connection and the direct bus interface it is not
necessary with a further check on the data. With traditional safety systems it may be necessary with test pulses etc. on all inputs to ensure
that they are functional, but this is not necessary with a direct bus interface, because the connections between the safety devices and the bus
interface are not taken out of the device so that short circuits etc. are not possible.
The receiver may be made redundant and diverse simply by using two PLC's from two different vendors programmed by two different programmers.
In this way, it is very unlikely that the two channels have the same hardware and/or software bugs. Because of the producer/consumer model and
the fast polling system, which ensures that all data or the lack of a poll answer are received exactly simultaneously in the two PLC's, it is very
easy to combine the outputs from the two PLC in one or more standard safety/emergency-stop relays with positive-guided contacts. It is also very
easy to supervise the two PLC's and programs and generate an alarm if only one PLC sets an output low.
It may be possible for telegrams to:
- get lost
- appear repeatedly
- be inserted additionally
- appear in the wrong order
- appear as another telegram (masquerading)
- be delayed
- contain destroyed data
The 22-bit CRC detect destroyed data as well as communication errors, and Max-i uses a transmitter signature with a 7-bit Hamming check on all
telegrams to protect against masquerading (point 5) and further increase the probability for error detection beyond what is possible with the CRC check
alone. Max-i also uses a telegram serial number on all safety-related telegrams. This protects against the first four points and further improves the
safety against masquerading. In this way, it becomes possible to combine safety devices and normal devices on the same bus!
The response time to an event may be less than 500 µS with a 120 m bus depending on any debounce circuits, however,
the safety depends on the standard time-out system. If a PLC has not received data from a safety device for a given amount of time,
it must shut down the system. This mechanism also protects against delayed telegrams. To increase the efficiency, Max-i has a heartbeat timer, which
ensures that all values are transmitted at least at regular time intervals from 10 millisecond to 10 minutes. It is therefore not necessary to poll the
values to find out if a device is alive.
Because Max-i is a pure producer/consumer system, it has no acknowledge telegrams. It has no meaning to acknowledge a telegram, which can be
received and utilized by more devices. If the issued telegram is not received, but transmitted correctly, the transmitter anyway cannot do
anything, even if there were an acknowledge telegram - except for logging the loss of telegram for later use when the communication channel (the
bus) becomes functional again. However, due to the telegram serial number the loss of telegram will be detected the next time the data are polled or
The telegram serial number also protects against hacking as this is detected when the original device transmits its data (heartbeat)
with an "old" serial number. The only way to fool the system is to transmit so many hacking telegrams that the counter wraps around, but due to the
babling idiot protection this is a very uncertain method, which is also quite easy to detect. Hacking protection is essential for burglary alarm
Max-i and PNS has been designed to enable an unambiguous and loss less conversion between XML (Extensible Markup Language) and the compact
fieldbus protocol. Even the shortest Max-i telegrams contain enough information - like for example the data type - for a complete conversion to XML.
If for example a button with the PNS number AV8SFB12EH101 is pressed this may cause the following XML telegram:
<siteArea name="Avedoere 8">
<ProductionLine name="Solid fuel boiler 12">
<equipment name="Emergency stop 101">
<function name="command 1" value="10B" time="26.13:27:15.598"/>
The time is counted directly in days, hours, minutes, seconds and milliseconds. If the bus length is 1 km, Max-i is able to transfer approximately
350 of these telegrams per second, which corresponds to the maximum telegram rate for most SCADA systems and databases.
of XML is that it is a very simple text based language, which may be used on many different platforms and generated very easily. Unlike traditional
method based systems, XML is self-describing with a name and description for each value and meta data telling how the telegram shall be interpreted. It
is therefore not necessary to have an exact number of arguments and to supply them in a precise order, so the various applications may be much
looser coupled than with for example DCOM (Distributed Component Object Model). Because each value has a name, XML fits perfectly together with
fieldbus systems using the producer/consumer model like Max-i and CAN. Both XML and Max-i has a variable data length, but without a data length
This page is modified May 19th 2015, but it may not comply 100% with the specification.
Please download the specification for the most up to date data.