RS-232 Communication

"Recommended Standard 232", more commonly known as RS-232, is an asynchronous, serial communication protocol established in 1960. Originally, it was commonly used for connections to modems, printers, mice, and other peripheral devices.

Like many other serial communication interfaces, RS-232 is not capable of real-time communication like you would get with, say, EtherCAT. However, you are guaranteed a fixed communication speed, or baud rate, unlike EtherCAT where max speed changes based on network traffic.

It’s also important to note that unlike the very similar RS-485 protocol, RS-232 is not suited to accommodate a network of three or more devices; it is solely for communication between two devices.


How RS-232 works

Most simply, devices communicating over RS-232 communicate over two sets of wires within a cable. Outgoing signals are Tx signals (transmitting) and incoming signals are Rx signals (receiving). These output signals are usually between ±5V and ±25V. To account for voltage drops along lengths of cable, the receiving device will interpret any voltage above +3V as a 0 and any voltage below -3V as a 1, simple binary communication.

What does the Data look like?

An RS-232 Transmission Signal broken down by bit

RS-232 transmissions are composed of several bits. First there is the start bit, which lets the receiving device know that it’s about to receive data. This is important because RS-232 is an asynchronous protocol, so it “primes” the receiving device to read the following data with the correct timing.

Next come the data bits, which can be anywhere from 5 to 9 bits, though 8 bits is most common. This is where the meat of the message is.

After the data bits come the parity bit, which is a small but fallible line of defense to check for errors. The parity bit specifies whether the number of data bits is even (0) or odd (1). The system will identify an error if it read an even number of data bits before an odd parity bit or vice versa. However, the obvious flaw with this system is that if 2, 4, or 6 data bits are misread, then the total number would still be read correctly as odd or even.

Last is the stop bit, which specifies either 1, 1.5, or 2 bit periods before the next start bit can be transmitted.

RS-232 vs USB

Small USB cable end next to larger d-sub cable end, both blue

USB interface (left) next to RS-232 interface (right)

Once upon a time, RS-232 ports were standard on most laptops and PC’s. However, USB has all but completely phased out RS-232 on computers since its introduction in 1996. But RS-232 still continues to be used with servers, CNC equipment, and of course servo drives.

RS-232 uses higher voltage differentials than USB, so naturally that results in higher power consumption. The D-sub connectors that our standard for RS-232 are much larger than a standard USB port, and considerably larger than a USB-C port.

But RS-232 is not without its advantages over USB. Because it uses larger voltage swings, RS-232 is less susceptible to EMI and can properly run over cable lengths of approximately 15 meters (3 to 5 times longer than what’s possible with USB).

More Information

  • In the past, common interface that allowed users to connect to the drive using standard serial ports built into their PC without needing to buy additional hardware
  • Today, low cost USB to RS-232 converters are readily available in the marketplace.

ADVANCED Motion Controls' Capabilities

  • Many existing drive designs use RS-232 as the configuration interface. See available models.
  • 115.2 kbit/sec max speed
  • 1.3 msec message time

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