Serial grounds (again)

[Andrew S]

I'm designing a controller that has a serial interface and I want to connect that serial interface to a BRX. The two devices share the same (24VDC) supply, therefore they have a common ground. Serial cable length between the units is less than 12 inches.

I've read that the BRX logic-ground (on the serial port) is connected to power-ground. I also believe that for DC a single ground should be run to each device. This is presentlt accomplished via the power ground. My concern is whether or not to connect the serial ground too. I would seem that I should not, in order to preserve the single-ground rule, but I am also wanting to make sure my serial connection has a solid ground, so I'd like to use it if possible. One other thing I thought of is that if I do use the serial ground I'll have the flexibility to have the two devices physically separated in the future, which would be useful.

Thoughts?

Andrew.

[BobO]

RS232 is particularly fickle with regard to grounds. RS485 not at all. The problem of RS232 is that it may work today and not tomorrow. Grounding generally resolves that.

The BX-SERIO's ports are all fully isolated, if that is of concern.

[ADC Product Engineer]

Grounding vs bonding.  There is a difference.

Grounding is to drain excess amperage and noise.

Bonding is to get everything on the same ground plane.

What you really want in this case is bonding.  As long as everything is attached to the same grounding point, the serial ground connections will likely not make a difference.  It is very small wire.  Hopefully your ground wire is much bigger.  The lowest impedance path (the larger wire) will be the one that is used for the majority of noise and excess current.   By not bonding the device, the RS-232 conductor will try to carry any excess current and that is a very bad thing.

[Andrew S]

That makes perfect sense. Thank you.

[BobO]

By not bonding the device, the RS-232 conductor will try to carry any excess current and that is a very bad thing.

It works...until it doesn't. Sometimes permanently.