RS-232 is an asynchronous communications method (a marching band must be “in syncâ€
with each other so that when one steps they all step. They are asynchronous in that they follow the band leader to keep their timing).
We use a binary system to transmit our data in the ASCII format. PLCs serial port is used for transmission/reception of the data, it works by sending/receiving a voltage, With RS232, normally, a 1 bit is represented by a voltage -12 V, and a 0 by a voltage +12 V. (The voltage between +/- 3 volts is considered There are 2 types of RS-232 devices.)
DTE – Data Terminal Equipment and a common example is a computer.
DCE – Data Communications Equipment and a common example is a modem.
PLC may be either a DTE or DCE device.
When plc and external device are both DTE, (or both DCE) devices they can’t talk to each other. The solution is to use a null-modem connection.
Usually, The plc is DTE and the external device is DCE.
Using RS-232 with PLC
Some manufacturers include RS-232 communication capability in the main processor. Some use the “programming port†for this. Others require a special module to “talk RS-232†with an external device.
External device may be an operator interface, an external computer, a motor controller, a robot, a vision system, etc.
To communicate via RS-232 we have to setup:
1. Where, in data memory, will we store the data to be sent?
2. Where, in data memory, will we put the data we receive from the external device?