[Info-vax] grounded MMJ cables

[Scott Dorsey]

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

-- 
"C'est un Nagra. C'est suisse, et tres, tres precis."