On 02/02/2018 03:13 PM, Fred Cisin via cctalk wrote: >> they might have different ground systems. If so, grounding both ends >> might produce a LARGE current through the cable, possibly enough to be >> hazardous. > > OB_Ignorant_Question: Is that the reason why RS232 DB25 has both pin 1 > and pin 7? >
I don't think so--pin 1 is basically a "protective ground", where pin 7 is a "signal ground". Consider an ordinary telco subscriber line. 2 wires, but there's also some sort of carbon block surge arrestor connected to an earthed ground at the subscriber's site. It may be connected to the AC protective ground (i.e. third prong in an outlet) or it may be separate. In my home, it was a wire attached to a copper rod driven into the earth. After a tree was uprooted and the cable unearthed, I notified my telco about it, was issued a service ticket--and nothing happened. I coiled up the ground wire neatly and buried it. It probably doesn't matter because my telephone line is buried its entire length back to the RT. It used to be that this wire was connected to the finger stop on old dial telephones. In any case, it isn't a signal conductor. --Chuck
