Proper Grounding; Table C-6. Eia-530 Interface Receiver - Motorola MVME2603-1121A Installation And Use Manual

Table C-6. EIA-530 Interface Receiver Characteristics
Differential input voltage
Input offset voltage
Differential input high threshold voltage
Differential input low threshold voltage
Input hysteresis
Input impedance (−15V < V

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 Table C-1 and Table C-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.
Parameter
< +15V)
in
Serial Interconnections
Value
Minimum Maximum
±12
±12
200
−200
1.0
3000 7000
C
Unit
V
V
mV
V
V
Ω
C-9