Serial Communications

Serial communications are still used extensively in automation despite the lack of serial ports on most computers. It used to be about the only way you could connect to and program controllers, whether PLC, DCS or others.

From the Primer:

Serial communications are strings of digital ones and zeroes sent over a single wire. They can alternate between sending and receiving data or have a dedicated line for each signal. The protocols for the data sent across the lines can vary widely but a few of the common types of serial communications are RS232, RS422 and RS485. The RS in these designations is an acronym for “recommended standard” and does not describe the actual communication protocol being used.

RS232 communications typically use separate send and receive lines. These are labeled as TX for transmit and RX for receive. They can also use other lines such as CTS and CTR for clear to send and clear to receive as a traffic control or hardware handshaking method. There are a number of parameters such as baud rate (the speed of the communication or bit rate), number of bits per character (7 or 8), whether or not a “stop” bit is used and whether the CTS and CTR lines are used (hardware handshaking). RS 232 signals are a common protocol used between computer terminals and various manufacturer’s control platforms. A 9 pin serial port is often included on a computer or control system which makes it a convenient tool for downloading programming into control devices. The TX and RX lines can either connect to the same pin numbers on each end or connect RX-TX and TX-RX which is known as a “null-modem” configuration.

RS422 and RS485 communications use a pair of twisted wires to carry the transmit and receive signals bi-directionally. For this reason they are often referred to a “twisted pair” communication. RS422 and RS485 can be used over a much longer distance and at a much higher data rate than RS232. RS422 is a multi-drop configuration whereas RS485 is a multi-point or “daisy-chain” configuration. These are often referred to as balanced or differential signaling.

This was information that would have been a good thing to know in my pre-internet days. It’s much easier to find info like this now, you can even do it with your phone… kind of scary…


Electrical Engineer and business owner from the Nashville, Tennessee area. I also play music, Chess and Go.

1 Comment on “Serial Communications

  1. A couple comments:
    RS-422 and RS-485 can be used over longer distances, faster because they use differential signaling (e.g. 4-wire RS422 has Rx+, Rx-, Tx+, Rx-) and lower voltages than RS-232.

    Over long distances, RS-422/485 need termination resistances at both ends (typically 120 Ohm, like the CAN bus).

    You do not have to use twisted pair wiring with RS-422/485 (for example, many RS-422 encoder cables do not), but twisted pair wiring will improve noise rejection.

    Serial ports still live in the USB world, not only through USB/Serial converters, but also because many USB devices use the USB port as a virtual serial port.