DS2/DS4 Hardware Manual
Page 23
LK13-15 Differential Pair Termination Links
Differential communications pairs should be terminated to prevent reflections from either end of the
communications cables. Each pair should be terminated at each end of the wire run with a resistance that
matches the impedance of the cable.
The AIM104-COM4 is capable of terminating each of the differential pairs on channels A and B using links
LK13-15. Two types of termination network can be configured: line matching terminators and pull apart
resistors.
Line Matching Terminators
: 124 ohm resistors can be linked across each communications pair. This will be a
fair match with twisted pair wiring. These should be configured only on boards at either end of the
wiring run.
Pull Apart Resistors
: The AIM104-COM4 is also able to pull each line of a communications pair to 5V
or 0V through a 4K7 resistor. This facility is primarily intended for use in RS422 multi-drop or
RS485 systems to ensure that the communications pair floats to a known state when no transmitters
are active.
Normally, a communications pair should be pulled apart in the inactive state ("Pulled Inactive"). This mimics
the condition where no character is being transmitted from the UART. It is however possible for the AIM104-
COM4 to pull the communications pair in either direction. Care should be taken when using the pull-apart
resistors in systems with multiple pieces of equipment on the same pair as each fitted pull apart network will
load the line driver devices. Reduce the number of fitted resistor networks to prevent this causing problems.
Each differential pair on the RS-485 ports can be terminated in six ways according to the following table:
Link Configuration Circuit
Configuration
Description
A
B
C
D
Unterminated
Line Driver connects directly to the
differential pair.
A
B
C
D
Terminated
Differential pair is terminated with 124
Ω
resistance between signal lines
A
B
C
D
Pulled Inactive
Differential pair is pulled apart to mimic
no character being transmitted from the
UART.
A
B
C
D
Pulled Inactive and Terminated
Differential pair is terminated with 124
Ω
and is pulled apart to mimic no character
being transmitted.
