[Info-vax] grounded MMJ cables

[chris]

On 07/18/21 14:06, Scott Dorsey wrote:
> chris<chris-nospam at tridac.net>  wrote:
>> 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
>> about standards...
>
> Correct.  There is no equivalent to pin 1 on an MMJ connector, only to
> pin 7.  Therefore pin 1 on the DB-25 side is brought out to a lead
> that can be connected to chassis shield.
>
> Whether or not you need it depends on your installation and what you are
> doing.  Everything should have one and only one connection to anything
> else through the ground net.  The manual for the device may discuss it.
> --scott
>


The only reason I can think of for the separate screen wire, apart
for emc reasons, is to ensure that both ends chassis are at the same
potential. The careless may use a 2 wire outlet at one end or the
other and switchers always have leakage, so a good safety feature...