Interface standards

Interface standards

Over the course of time certain interfacing standards have been generated by industry in order to make
communication between systems from two different manufacturers more simple. These standards typically
defined the communications medium, transmission voltages, speed of communication, (baud rate), and
maximum distance sometimes related to speed.
The first such real standard was RS232. This was written by the Electronic Industries Association (EIA)
in the USA and was fairly complex, including the definition of 22 terminations between the two interfaces.
Today of course, few people use more than four wires and a screen for RS232 communication, using a subset
of the original, RS232C. Most, if not all, PLCs today have an RS232 interface, for instance:
AEG Modicon - Modbus
Allen Bradley - Data Highway
Siemens - 3964R (CP525)
Some RS232 protocols today also allow for the addressing of nodes, thus providing for network
communication from a standard R5232 interface. The main advantage of this being cost as RS232
interfaces are very simple and thus cheap, plus of course the fact that most PCs, which are used more and
more in automation systems today, have at least one PS232 (serial) interface as a standard.
RS232, however, has some limitations. Distance, a maximum of 50 ft by definition (without modems) is
a major problem, and a maximum transmission speed of 19200 bits per second (baud). This led to the
development of several other interface standards, the most common in the PLC industry being RS485,
IEEE 802.3 (CSMA/CD) and IEEE 802.4 (token bus).