I want to connect 2 devices together using RS232. I initially thought both devices used the power ground as the reference signal for their RS232 (they're powered by the same PSU) so I designed my RS232 cable just using Tx and Rx. However, after some testing and some bad RS232 communication, I learned that one of the devices actually uses case ground (earth ground) as the reference signal. If I connect the RS232 reference signal pin, the communication is flawless, but I want to avoid connecting my floating ground to earth ground (I believe this is good safety practice). Is there someway I can create an isolated reference signal for the two devices to use?
Electronic – RS232 Grounding question
groundrs232
Related Solutions
When we needed to do something like that where I worked many years ago, when testing power controllers that had a direct mains connection (no transformer) under load with a scope I specified a high-power isolation transformer in an "earth free" test area. The setup cost a lot of money, but was necessary for safety reasons, and worked very well.
People sometimes remove the scope ground connection in those circumstances, but it isn't a good idea.
In your case, I'd use an isolation transformer and an earth-free area.
It will depend of the software that you're using.
A) In my case, as I develop my own PC software, I've never used other pins, only RxD, TxD and GND. So, in this case you can left other pins open. This is called as null modem without handshaking
B) Yes (assuming GND is being considered)
C) Again, it will depend of the software on PC side. Some softwares you can configure to not take care of hardware control. Others will require RTS/CTS handshaking, so you can do a null modem with loop back handshaking
Source of the pictures: RS232 serial null modem cable wiring
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Best Answer
You need to wire the signal ground (Pin 5 on DB-9 RS232) always.
Using only TX and RX is simply not a valid wiring IMHO. If it works, it's by accident and you are relying on the having the exact same ground potential on both sides. Which is ok for an internal UART connection within a device, but not between two separate devices.
If this causes a ground loops between two devices, or unwanted potential shifts, using fully opto-isolated RS232 as mentioned earlier is the only correct way.