Stratus Engineering

RS232 to RS422/485 Conversion Cable Connection Requires No External Electronics or Power

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In many cases where an RS232 to RS422/RS485 connection is required it is possible to perform the signaling conversion without an external adapter and power supply using the interface connections shown below.

rs232-rs422-rs485-conversion-cable

Why does this work?

This arrangement provides connections that effectively satisfy the voltage level requirements of the RS232 and RS422/RS485 receivers for the interconnected devices as follows:

  1. When the RS232 device transmits logic “1” its driver applies a -5V signal to the “TX” line, which the RS422/RS485 receiver sees as a differential +5V (0V – (-5V)) or logic “1”.
  2. When the RS232 device transmits logic “0” its driver applies a +5V signal to the “TX” line, which the RS422/RS485 receiver sees as a differential -5V (-5V – 0V) or logic “0”.
  3. When the RS422/RS485 device transmits logic “1” its driver applies a 0V signal to the “TX+” line, which the RS232 receiver sees as 0V or logic “1”. Although RS232 receivers expect a negative voltage for logic “1” their logic switching threshold is actually +1.5V rather than 0V so that they interpret a 0V input as logic “1”.
  4. When the RS422/RS485 device transmits logic “0” its driver applies a +5V signal to the “TX-” line, which the RS232 receiver sees as +5V or logic “0”.

Limitations

While this somewhat unorthodox conversion eliminates the external RS232 to RS422/RS485 adapter electronics, there are some limitations to its use:

  1. When using this connection, cable lengths and baud rates are limited to those that would work with a standard RS232 channel. Applications that use RS232 to RS422/RS485 conversion to increase allowed cable length generally won’t work in this configuration.
  2. There must be a good ground connection between the two devices.

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To learn more about Stratus Engineering’s design services, please contact us via email or call us at (858) 663-1841.

Bob Kirstein