Motorola MVME187 Installation Manual page 163

Levels of Implementation
Proper Grounding
Another 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 the chassis to be in
compliance with the electrical code.
E
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 Figure E-1 shows no connection for pin 1.
Normally, pin 7 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.
E-7