Motorola MVME2600 manual Proper Grounding, Table B-6. EIA-530 Interface Receiver Characteristics

Proper Grounding

Table B-6. EIA-530 Interface Receiver Characteristics

Parameter	Value		Unit
	Minimum	Maximum
Differential input voltage		±12	V
Input offset voltage		±12	V
Differential input high threshold voltage		200	mV
Differential input low threshold voltage		−200	V
Input hysteresis	1.0		V
Input impedance (−15V < Vin < +15V)	3000	7000	Ω

B

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 B-1and Table B-4show 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.

http://www.motorola.com/computer/literature B-9