Proper Grounding - Motorola MVME167 Series User Manual

EIA-232-D Interconnections
A

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.
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
A-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.
A-6 MVME167 Single Board Computer User's Manual