RS423 serial information

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• Introduction to RS423
• Single ended variable slew rate
• Electrical characteristics of RS423

Introduction to RS423

The RS423 standard is one of the lesser known serial communication standards. Its older brother RS232 is widely known because serial ports with this interface are present on almost all computer systems. RS422 and RS485 are differential which makes them useful in applications where noise immunity is an issue, like in industrial applications. The single ended RS423 standard sits somewhere in between these standards as an enhancement of RS232 with longer cable lengths and higher allowed data rates.

Although RS423 is currently not widely implemented, it has seen a broad usage in the late eighties of the previous century because of its backward compatibility with RS232. Hewlett Packard shipped their computers with a serial interface capable of communicating on both RS232 and RS423 levels and Digital Equipment Corporation used the RS423 signal levels on their DECConnect MMJ serial interface standard. Because RS232 and RS423 are both single ended serial communication standards with comparable signal levels, it was possible to use these RS423 interfaces in both pure RS423 applications, and mixed RS232/RS423 situations.

Single ended variable slew rate

Although RS423 is a single ended serial communication interface just like RS232, the standard allows higher data rates and cable lengths. This is achieved by introducing an adjustable slew-rate. With RS232, the slew-rate is fixed at a speed of 30 V/µs. RS423 has a variable slew-rate which can be set dependent on cable length en data rate. This adds noise immunity to the interface. On longer cable lengths where noise can be an issue, a lower maximum data rate can be set and the slew-rate is changed accordingly. RS422 and RS485 which are both differential serial interfaces do not need such a system of noise immunity.

The maximum allowed cable length with RS423 is 1200 meter, just as with RS422 and RS485. The maximum data rate is 100 kbps which is only five times faster than default RS232 and much slower than possible with RS422 and RS485. Detailed information of the interface characteristics of RS423 can be found in the serial interface comparison table.