On 07/18/21 12:08, Phillip Helbig (undress to reply) wrote:
> In article<sd10f5$pd6$1...@gioia.aioe.org
>> On 07/18/21 09:35, Phillip Helbig (undress to reply) wrote:
>>> In article<sd0n91$i5o$1...@tncsrv09.home.tnetconsulting.net
>, Grant Taylor
>>>> On 7/17/21 11:22 PM, Phillip Helbig (undress to reply) wrote:
>>>>> I'm testing out some MMJ cables (useful for console cables) and
>>>>> notice that some (all of which are round rather than flat) have an
>>>>> additional wire outside of the plug at each end, which looks like it
>>>>> might be intended as ground. What's the purpose of that?
>>>> How long is it? I'd expect it to be 3-9 inches if it's meant to be a
>>>> ground wire that is attached to something else on the chassis.
>>> Yes, about 5 inches. It has an eyelet at the end, so one could connect
>>> it to the chassis via a screw.
>>>> I would also have significant worries about ground loops and / or sneak
>>>> currents. Both of which can be quite dangerous from an electrical and
>>>> safety perspective. As in more amperage than the wire can safely handle
>>>> turning the wire into a heater that can melt insulation and start fires.
>>> But is that an issue when using a console cable?
>> Not usually from an interference to the cable point of view and iirc,
>> the added wire is at one end only, which would not enable earth loops.
> I have seen those, but the two I checked today have it at both ends.
Ok, but the one I have has just one, but maybe that was third party
The frame or chassis ground and signal grounds are separate pins
on a 25 pin RS232 layout, even though they may be joined inside the
equipment. If they joined, it's often with a low value resistor, say
100 ohms, to prevent excessive earth loop currents. Don't know the
circuitry for a VT220, for example, but DEC would have done the job
right, both from an emc and safety points of view. They were obsessive