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What is a protocol?
Copyright © Romilly Bowden 1998.
"Protocol" - OED, 3rd edition (among other meanings):
- the formulary of the etiquette to be observed by the Head of State in official ceremonies
- rigid prescription or observance of precedence and deference to rank ....
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 Descriptions | "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 | IEC1158-2 (H1) | IEC1158-2 (H2) | |
|---|---|---|---|---|---|
| Signal (typical) | ±12V | ±5V | 0.5V pk-pk | 0.9V pk-pk | 7V pk-pk |
| Coding | NRZ | NRZ | FSK, 1200/2200Hz | Manchester | Manchester |
| Balanced line and matched terminations? | No | Yes | No | Yes | Yes |
| Speed (bits/sec) | up to 115k | up to 1M | 1.2k | 31.25k | 1M, 2.5M |
| Distance* | 16m | 1200m | 2000m | 1900m | 750m, 500m |
| Multidrop? | No | Yes | Yes | Yes | Yes |
| Spurs? | No | Yes** | Yes | Yes | No |
| Power on the bus? | No | No | Yes | Yes | Yes |
| Intrinsic safety? | No | No | 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" to find restrictions on speed 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 | Data types | Meaning of data |
|---|---|---|---|
| Modbus | RS-232, RS-485 (9.6 - 38.4kbps) | Integers, bits * | Standard commands. Registers, status, coils |
| HART | HART FSK (1.2kbps) | Integers, bits, floating point, date, text | Universal and common-practice commands.
Device Descriptions for extensions |
| Profibus DP, FMS | RS-485 (9.6k - 12Mbps) | ? | Profiles for standard devices |
| Profibus PA | IEC1158-2 (H1) | ? | Profiles for standard devices. AI and AO function blocks |
| WorldFIP | IEC1158-2 (H1) | Integers, bits, floating point, text | Companion standards for standard devices. |
| FOUNDATION Fieldbus | IEC1158-2 (H1) | Integers, bits, floating point, date, time, text | Standard function blocks for control in the field. Device Descriptions for extensions |
* 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 always welcome. E-mail me.
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