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What is a protocol?

Copyright © Romilly Bowden 1998, 2009.

"Protocol" - OED, 3rd edition (among other meanings):

Relating to digital communications, a protocol is a set of rules which ensure orderly passing of useful information between two or more devices.

Questions the protocol rules may cover include:

Question Topics OSI layer
How do we make the connection? Plugs, sockets, cable Physical
What signals can I send? Voltage, current, frequency Physical
How do I address a message? None (point-to-point), numerical address, tag DataLink
When can I send a message? Access rules: master-slave, token-passing, collision-detection DataLink
What messages can I send? Coding: bits, characters, parity DataLink
Data types: bits, integers, floating point, text Application
What does a message mean? Standard functions Application
Function blocks, Device Description (DD), Device Type Manager (DTM) "User"*

* The "User layer" is not part of the ISO OSI ("Open Systems Interconnection") protocol model, but its features are important for interoperability.  The full OSI "stack" also includes other layers to deal with more complex environments, not usually required in an industrial network.


Physical layers

Various electrical (and non-electrical) physical layer specifications exist.  Some of the more important for industrial applications are:

RS-232 RS-422/485 HART FSK IEEE802.15.4
(FF H1 & Profibus PA)
Signal (typical) ±12V ±5V 0.5V pk-pk 1 to 10mW
2.4GHz radio
0.9V pk-pk ±2.5V/1V
Coding NRZ NRZ FSK, 1200/2200Hz O-QPSK Manchester TCP/UDP/IP
Balanced line and matched terminations? No Yes No n/a Yes Yes
Speed (bits/sec) up to 115k up to 1M 1.2k 250k 31.25k 10M/100M
Distance* 16m 1200m 2000m 100 to 200m 1900m 500/100m
Multidrop? No Yes Yes Yes Yes ?
Spurs? No Yes** Yes n/a Yes No
Power on the bus? No No Yes n/a Yes No
Intrinsic safety? No No Yes Yes Yes No

* Longer distances may well be possible, using higher quality cables or repeaters.
** (depending on speed)


Some protocols

This table shows a selection of interesting industrial communication protocols.  Look at the "physical layer" (refer to the table above) to find restrictions on topology and cable length.  Look at the "data types" and "meaning of data" to see what sort of information each protocol is good at.

Protocol Physical layer Speed Data types Meaning of data
Modbus RS-232, RS-485 9.6 - 38.4kbps Integers, bits * Standard commands. Registers, status, coils
HART FSK FSK 1.2kbps Integers, bits, floating point, time, date, text, trends, alarms, events, status Universal and common-practice commands.
Device Description (DD) for extensions
WirelessHART IEEE802.15.4250kbps
Profibus DP, FMS RS-485 9.6k - 12Mbps  ? Profiles for standard devices
Profibus PA IEC1158-2 (H1) 31.25kbps  ? Profiles for standard devices. AI and AO function blocks. DD, FDT/DTM
WorldFIP Integers, bits, floating point, text Companion standards for standard devices.
FOUNDATION Fieldbus H1 Integers, bits, floating point, date, time, text, alarms, events Standard function blocks for control in the field. DD for extensions
FOUNDATION Fieldbus HSE Ethernet 10 - 100Mbps

* Floating point and text extensions exist, but are non-standard and not always compatible between devices.

Warning! This note offers only a very simplified and incomplete view of the protocols listed.  Always check with suppliers for the latest and most accurate information!

Corrections or comments are always welcome.  E-mail me.

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