Crossing RS-232 Signals... dangerous?

Hello everybody of the forum!

I have been working a lot with RS-232 lately and had a fundamental type of question.

I understand most or all of the signals involved with RS-232 (TX, RX, RTS, CTS, DSR, DTS, DCD, and RI on older systems, etc.) I know that many applications call for a null modem to cross TX and RX signals between certain device connections (like DTE to DTE connections for example.)

My question is - are there any situations where it is dangerous to cross signals, or could potentially cause damage to equipment by using the wrong cable?

For example, I was in the field looking to connect to a DB25 connector via a DB9 to DB25 RS-232 convertor cable. Some manufacturers use cables that physically look the same (DB9 on one end, DB25 on the other) where the pins will not match up with the RS-232 pinout. Is it possible that using an incorrect connector in such a situation would damage the customer's equipment? My best guess is that the inputs would be isolated (optically for example), but I figured I would ask the panel of experts for real-world situations and experience.

Thank you!

i cannot say in regards to the rs232, but i ill say that i had a bad experience with rs485 and ethernet once. the technician connected his laptop to a slc 5/03 using an ethernet cable and the plc power supply kept shutting off. when we used the correct cable, everything was ok. no damage to the ethernet port o the laptop, we got lucky.
james

Generally, using RS232 is safe if using cables designed for RS232. I have screwed up connections in the past without causing damage. The one situation I remember where damage could occur was a servo drive manufacturer that decided they would use one DB9 port to house both RS485/422 and RS232 signals. You had to wire your connector to match the style of signaling you wanted. If you used a generic RS232 cable on their port, it would burn out the chip and it would need to go in for repair. That was MANY years ago and is the only time I saw an issue with RS232 wiring.

Regarding the cables you use, just ensure that they are wired for valid RS232 or RS232 null modem connections and you should be fine.

Not sure if I am an expert but anytime you hook up to an unknown you are taking a chance, for example a DH485 port on a SLC 5/xx has 24vdc on pin two, the connection is a standard RJ45 so it looks like a standard Ethernet port but if you plug in your Ethernet cable from your PC to it you will no longer have a PC or Ethernet card, I make some radio cables for a member here and they are custom, the are a standard HD15 pin cable and they I think also have 24VDC... I think they are power up a modem

But the simple answer is, Yes it can be dangerous.

Not plc stuff but I remember Fanuc RH series robots had +24 on one of the db25 pins to power the external floppy drive. I think it was pin 25.

I always shut the power off when swapping pins or disconnected the cable when swapping pins. I tried to stay away from soldering and used the crimp style and that pin pusher to swap pins in the connector.

I don't think I had a problem with any 232 device when I follow these notes.

When I had the TX going to TX or RX going to RX it did no damage to devices.

jim3846 said:

i always shut the power off when swapping pins or disconnected the cable when swapping pins. I tried to stay away from soldering and used the crimp style and that pin pusher to swap pins in the connector.

I don't think i had a problem with any 232 device when i follow these notes.

When i had the tx going to tx or rx going to rx it did no damage to devices.

Click to expand...

+1

I am not expert either just old timer---rs 232 is just two wires if swapped it won't work but no damage. before everyone goes off on me, I am say only the two wires TX and RX, sometimes a third wire is present as a shield or drain, sometimes I have seen ground or drain connected both ends with rs232 others not.

It's funny doing one today got a new hmi from OEM(old hmi did same guessing game with high price parts) connected did not work connected ground or drain as they instructed still did not work old plc oem sends to another board and re-transmits serial connection so don't know which is bad PLC or re-transmit board.

Can't get into any thing OEM has it locked and they really like their products, many $$$

I have one of those old RS232 configurable dongles so certain lines can be changed via dip switches, the ones that are fixed are the common & screen, used it for many years & set switches while connected in all manner of combinations & not once did I destroy a 232 port, 422 & 485 do seem to be a little more touchy not that I have ever done it wrong & lost one. so in my experience, never had a problem.

Can't damage 232 by connecting wrong terminal amongst themselves. Can cause damage if you introduce other voltages.

parky said:

I have one of those old RS232 configurable dongles so certain lines can be changed via dip switches, the ones that are fixed are the common & screen, used it for many years & set switches while connected in all manner of combinations & not once did I destroy a 232 port

Click to expand...

Exact same experience on my end.

RS-485 half duplex driver lines should connect (+) to (+), (-) to (-) or A to A, B to B, but are frequently connected backwards because labeling by vendors is not standardized.

Backwards connections of the drivers will NOT damage the drivers, but communications will never work.