MC75172B_06 Datasheet, PDF (10/13 Pages) ON Semiconductor – Quad EIA−485 Line Drivers with Three−State Outputs

MC75172B, MC75174B
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
above V_CC or below ground. These overshoots and
undershoots can disrupt the driver and/or receiver
operation, create false data, and in some cases damage
components on the bus.
En
TTL
D
R_T
#1
R
TTL
R
TTL TTL
#1
#2
5 “off” drivers (@ 0.06 U.L. each),
En
+8 receivers (@ 1.0 U.L. each) = 8.3 Unit Loads
TTL
D
R_T = 120 W at each end of the cable.
TTL
#2
En
D
#3
R
#3
En
TTL
D
#4
120 W
Twisted
Pair
En
TTL
D
R_T
#6
R
TTL
#6
TTL
R
#4
TTL
R
#8
R
TTL
#7
R
#5
TTL _TTL
En
D
#5
NOTES:
1.Terminating resistors R_T must be located at the physical ends of the cable.
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.
Figure 18. Typical EIA−485 System
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