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

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.

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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