RS232 Cable Shielding Opinions

[Petro]

Hi All,
I am curious what your opinions and recommendations are on where to put the Shield on RS232 cables.

Here are all the options that I have heard to put the cable shield.
Pin1 on the DB25
Pin7(signal gound) on the DB25
Attach to Housing or outer shell on DB25
Connect both Pin1 & Pin7 on DB25

Pin1 on DB25 seems to be somewhat common from what I have seen in documentation. I have seen sometimes where there was issues on the machine and touching the outer shell to the machine has created a small spark. So my thought would to connect to Pin7 would make the most sense as this is signal ground or the reference voltage. However I am not all to sure. Does it make a difference using one shield config over another for longer cables? Are Pin1 and Pin7 generally connected together on the machine side?

Thanks for your insight and comments!

[Ted Fergason]

Pin 1 is used for shielding, it’s also chassis ground.
Do not attach the shield to both machine and computer. One side or the other should be sufficient. Attaching to both sides on rare occasions has caused problems for me.
Pin 7 is the UART signal ground only.
Also if you expect stormy weather, it might be a good idea to disconnect the cable if not in use
A lighting strike on the building can blow out the UART.

[Strat]

As a no frills set up (2 to 3, 3 ~ 2 jump 4, 6 8, 20 to each other on both ends and tie off the braided shield to pin 7 on one plug only. But doing field service I needed the ready to send clear to send waiting for me and to just minmize any electrial noise more than anything.. A low rent way but I didn't want to spend a lot of time troubleshooting any ul/dl problems.
But that was cheating and I know it but it worked fine for me no matter the brand of control or machine..
LOL

[MTS]

Pin 1 on DB25 is connected to the shield. 6-8 to 4 and 20 to 1-6. This is on a 7 wire 25to9-pin cable. Never connect both ends to the shield, as it can give you trouble in the handshake signals, and even error in RTS/CTS signal.

[Petro]

One thing I noticed once was that a customer had a dektop PC hooked up to a machine and when they used there computer it would draw down the voltage through the ground wire of the PC. I actually cut off the ground plug off the PC to prove it. When using my laptop it did not do the same thing. So I am wondering if grounding also makes a little bit of a difference depending on whether a USB to serial device is used verses a serial card in a PCI or PCIE slot or laptop for that matter in regards to grounding and eliminating the amount of electrical noise as well.