Serial Digital Communications Overview

Introduction to digital communications in temperature and process control

Products which support digital communications allow a computer system to read process data, display it on the screen and to store it away. The link may also be configured to write values to the equipment, thus providing a flexible Machine Interface. Communications links may also be used to allow one controller to pass parameters such as setpoints directly to other controllers, allowing more complex control strategies to be implemented.

The data to be transmitted is broken up into small pieces known as Words which are then transmitted as a succession of changes of state. Each state, referred to as a Bit, has the same time duration. The number of bits sent per second is known as the Baud Rate. There are any ways in which the message content can be broken down into words and in order to be able to re-create the original message it is necessary for the receiving equipment to know how this was done and the rate at which the bits are being sent. The diagram shows several different ways in which the bits forming a word can be interpreted. This interpretation is described by the term “Protocol”.

The word Protocol is used to describe more than one element of the communications structure.

• Signal Levels. The voltages on the link wires and the number of wires employed : +/- 12 Volts relative to a common line +/- 200mV differential between the wires
• Data Coding. The structure of each byte:
  • 1 Start /7 Data / Even parity/1.5 Stop Bits
  • 1 Start /8 Data /No Parity/1 Stop Bit
• Data Structure. The way that the message is actually encoded and transmitted:
  • EIBisync
  • Modbus
  • J-Bus

RS485 Differential Mode signalling using either one or two pairs of wires +/- 200mV signal required at the receivers. Signalling levels at the transmitter of 0 and 5Volts It is apparent that there are similarities between RS422 and RS485. They are not, however, identical and the differences are important:

• RS422 – has a maximum of 10 Standard Loads per link. This is the limit to the number of Eurotherm instruments which can be connected onto a single link
• RS422 – always uses separate pairs of wires for communication in each direction
• RS485 – has a maximum of 32 Standard Loads per link. This is the limit to the number of Eurotherm instruments which can be connected onto a single link
• RS485 – allows EITHER two pairs of wires for communication OR a single pair of wires for transmission in both directions.

When used with two separate wire pairs, RS485 can thus be seen as a higher capability version of RS422 . When used with only a single pair of wires there must be co-operation between instruments at each end of the link in order to ensure that both ends do not try to use the wires at the same time. The equipment must be designed to operate in this way.

Instruments designed for two pairs of wires will not communicate correctly on one pair. In using RS485 communications all instruments in the link must be designed for the same wiring arrangement.