Post by chris
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
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.
"C'est un Nagra. C'est suisse, et tres, tres precis."