Motorola MVME187 User Manual

Levels of Implementation

E-7

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 

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.