MC75172B MC75174B

http://onsemi.com

11

Termination Resistors

Transmission line theory states that, in order to preserve

the shape and integrity of a waveform traveling along a

cable, the cable must be terminated in an impedance equal

to its characteristic impedance. In a system such as that

depicted in Figure 18, in which data can travel in both

directions, both physical ends of the cable must be

terminated. Stubs, leading to each receiver and driver,

should be as short as possible.

Leaving off the terminations will generally result in

reflections which can have amplitudes of several volts

undershoots can disrupt the driver and/or receiver

operation, create false data, and in some cases damage

components on the bus.

Figure 18. Typical EIA−485 System

2. Stubs should be as short as possible.

3. Circuit ground of all drivers and receivers must be connected via a dedicated wire within the cable.

Do not rely on chassis ground or power line ground.

#8

TTL

R

TTL

TTL

#3

TTL

R

En

#2

TTL

TTL

En

TTL

R

TTL

#1

R

TTL

#1

En

D

#3

D

#5

D

TTL

En

#6

R

TTL

R

En

#5

R

#6

D

#7

TTL

TTL

R

En

TTL

#4

D

5 “off” drivers (@ 0.06 U.L. each),

+8 receivers (@ 1.0 U.L. each) = 8.3 Unit Loads

#4

D

#2

120

Ω

Twisted

Pair