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Levels of Implementation
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-7
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