XLR cable for serial data + 5v?

[swinginguitar]

anyone done this? possible?

one pin for serial and the other for +5v both referenced to ground pin?

[Processaurus]

That's what a MIDI cable is, just a different connector.  You can build MIDI cables with bulk microphone cable.   I got some parts to make midi to XLR connectors for running MIDI through a snake, need to get around to trying it.

Watch your length though, after a certain amount of cable length the data can start getting weird, noise and cable capacitance adding up.  Digital standards specify maximum length cables that can be reliable used, I believe MIDI is 50', USB is 15', etc.  Balanced line standards like CAT5 can go much longer (300').

[swinginguitar]

do you tie each end of the ground to the local circuit ground, or is one not connected (wondering about ground loops etc).

Didn't realize that cat5 was balanced....is it the same sort of thing as balanced XLR where you flip one and then sum them back together at the other end?

[Processaurus]

do you tie each end of the ground to the local circuit ground, or is one not connected (wondering about ground loops etc).

What is the application?  It depends if you need the ground for the communication circuit, or if it is just the shield.  MIDI spec has the ground floating on the receiving end, for example.

Didn't realize that cat5 was balanced....is it the same sort of thing as balanced XLR where you flip one and then sum them back together at the other end?

Yes, I believe that is the deal with the 4 twisted pairs of wires, by twisting a pair together tightly, they freely pick up the same noise which you can then reject by subtracting one from the other.

[swinginguitar]

it's all just a plan right now, but:

application would be a box of momentary switches remotely controlling a box of relays (each box has a PIC...one way UART serial comm, +5v sent to remote box from the relay box for power)



the MIDI spec is what made me wonder about floating ground...when is it appropriate?

[Processaurus]

Ultimately you want there to be a single path from each device to earth ground, so with what you're describing there is no problem connecting the switch box to ground of the receiver relay box.  Actually, you need to have a ground connection, right, since you are supplying power through the XLR cable from the relay box?

[G. Hoffman]

do you tie each end of the ground to the local circuit ground, or is one not connected (wondering about ground loops etc).

Didn't realize that cat5 was balanced....is it the same sort of thing as balanced XLR where you flip one and then sum them back together at the other end?

MIDI standard has the shield conductor grounded at the sending device only, so you connect to both sides inside the cable, because it shouldn't be connected on the MIDI input device, and if both ends of the cable are connected to ground you can use the cable either way.  Or, to put that another way, in a MIDI cable Pin 2 is connected to the shield on both ends.  When you plug it in, it only gets grounded at the MIDI send connector, and the MIDI Input connector has Pin 2 floating.  The Opto isolates the input end, and you can be certain there won't be a ground loop.  (Assuming the manufacturers followed the standard, of course!)


Gabriel