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.
Why choose digital communication of analogue communication?
Digital Communications is preferable to analogue communications (e.g. 0-5V) because:
- There is no accuracy variation with distance
- No inaccuracy is introduced by the A-to-D and D-to-A conversions at each end
- Controllers which use Microprocessors have the data already available, formatted correctly
- Calibration of the retransmission components is not required
- One-to-one or One-to-Many communications links are possible Digital Communication encodes the data to be transmitted into two voltage states. Figure 1 shows how this can be done.
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:
The wiring is covered by a number of recognised standards and the ones used on Eurotherm equipment are:
RS232 +/-12 volts nominal (3V min,15Vmax) 3 Wires used, TRANSMIT (Tx), RECEIVE (Rx) and COMMON
This can only be used with a single device at each end of the wiring and is thus a One-to-One link
RS422 Differential Mode signalling using two pairs of wires, one pair in each direction +/- 200mV signal required at the receivers. Signalling levels at the transmitter of 0 and 5Volts Permits communication from one “master” device to many “slave” devices.
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.
Profibus® DP Fieldbus
Profibus communications are supported by a number of products from Eurotherm. This 'fieldbus’ system allows very high speed digital communications using an enhanced RS485 wiring technology, and has become a de facto standard in factory.
DeviceNet is a cost-effective communications link designed to replace hardwired I/O interconnection between industrial devices.