Description of Various Communications Standards

The increasing use of micro-processors within products in the process industry has brought increased flexibility to change user-related data as required during configuration, installation, and ongoing use.

In order to set up the various parameters, the device is usually provided with some form of digital communications whereby data can be transferred to and from the equipment, often using a personal computer. Real time data can also be accessed and transferred to and from the equipment as part of a larger system.

The process of communication between two devices is relatively straightforward, and can be likened to a conversation between two people - the mechanism is reliable and there are little or no difficulties in carrying on the conversation by speaking in turn as long as both parties speak the same language. However, it is clear that if one person wished to talk to a number of people, say by telephone, then a other factors need to be considered:
● Do they speak the same language？
● Does everyone have a unique telephone number？
● Do some instructions have to sent to everyone？
● Is the telephone conversation clear and reliable and if not what action is to be taken？
● Can more than one person talk at once or do they only talk in turn？

Problems can occur when attempting to establish communications unless a rigidly defined set of conditions is established and certain rules followed. These conditions and rules form the communications "protocol" and, when correctly implemented, ensures reliable data transfer can occur.

A formal communications protocol requires a significant amount of processing power and electronic components to ensure correct operation. A balance must be found within a given product between providing sufficient standardisation of communications facilities for normal use and keeping the overall cost of product design and manufacture at an appropriate level for the intended market.

As a means of clearly defining communications protocols, a "model" has been developed which splits the various aspects of a communication system into "layers".

A fully specified computer network protocol might have seven or eight layers, whilst the requirements of industrial systems can be accommodated within a simplified "model".

In a typical industrial application, the layers would include:
● Physical - the actual connection between the devices, e.g. RS232, RS485
● Data Link - the data link layer includes control of access (to the physical connection), the flow of data and the detection and correction of errors
● Network - the network layer controls routing of data and related tasks.
● Application - the application layer generates and receives data messages in a consistent way and provides the interface to other local functions (e.g. operator or equipment interface).

Status Instruments' products incorporate a range of communications facilities for many years. Most of these have been based on a Master-Slave mechanism utilising protocols derived from ANSI X3.28

Current Status Instruments developments include other widely accepted protocols - HART and Modbus, both well established in the process industry, each having benefits over the other in particular applications.


HART

The HART - "Highway Addressable Remote Transducer" interface was developed by Fisher Rosemount for communication with process signal transmitters and subsequently released as an open specification. Although originally conceived and developed as a "point to point" mechanism, it was enhanced to provide limited capabilities for a "multi-drop" digital-only mode.

Data is transferred to and from the remote device by means of a small A.C. signal superimposed onto a conventional 4 to 20 mA analogue signal. The A.C. signal is generated by frequency shift keying (f.s.k.) at 1200 baud (Bell 202 standard). Having an average value of zero, the f.s.k. signal causes no interference with the analogue signal. Master devices transmit voltage signals, while slave devices transmit current signals. Both master and slave devices receive data by means of voltage sensitive circuits ( in the case of the master, operating in conjunction with the loop load resistor).

HART is a master-slave protocol, but accommodates two masters. Up to 15 slave devices can be connected in a multi-drop configuration (for non intrinsically safe applications).

The protocol includes three groups of commands :
● Universal Commands - which provide functions that must be implemented in all field devices, such as "Read manufacturer and device type" or "Read Primary Variable (PV) and units".
● Common Practice Commands - which provide functions common to many field devices but not all. e.g. "Write transmitter range", "Perform Self-Test".
● Transmitter Specific Commands - these provide functions which are more or less unique to a particular type of field device, for example "Start, Stop or Clear Totaliser".