Proper Grounding - Motorola MVME162 User Manual

Serial Interconnections
A

Proper Grounding

An important subject to consider is the use of ground pins. There are two pins
labeled GND. Pin 7 is the signal ground and must be connected to the distant
device to complete the circuit. Pin 1 is the chassis ground, but it must be used
with care. The chassis is connected to the power ground through the green
wire in the power cord and must be connected to be in compliance with the
electrical code.
The problem is that when units are connected to different electrical outlets,
there may be several volts of difference in ground potential. If pin 1 of each
device is interconnected with the others via cable, several amperes of current
could result. This condition may not only be dangerous for the small wires in
a typical cable, but may also produce electrical noise that causes errors in data
transmission. That is why Tables A-1 and A-4 show no connection for pin 1.
Normally, pin 7 (signal ground) should only be connected to the chassis ground
at one point; if several terminals are used with one computer, the logical place
for that point is at the computer. The terminals should not have a connection
between the logic ground return and the chassis.
A-8 User's Manual