PCM-2074 EVB User Manual
SMSC PCM-2074
USER MANUAL
Revision 1.0 (03-31-06)
11
9.3
Offset Resistors
The function of the offset resistors is to prevent the differential voltage on the twisted pair cable, which
is the voltage between the positive signal and the negative signal, from going to zero during idle
periods when nothing is driving the line. To maintain stable operation, the RS485 transceivers require
a differential voltage of generally at least 200mV.
The offset resistors, which operate as a pair, supply this differential. One Offset Resistor biases the
positive signal toward Vcc, while the other biases the negative signal toward ground. The configuration
of the offset resistors within a CircLink node depends on how that node is connected within a network.
The offset resistors should be installed on the end nodes of a network and should be removed or
disabled on the middle nodes of a network containing three or more nodes. This is shown in
below. (In the PCM-2074, the offset resistors are configured using JP1 and JP2.).
Figure 9.3 Offset Resistors
Offset Resistor Configurations:
- Connect offset resistors on end-nodes
- Remove or disconnect offset resistors on middle nodes
CircLink Controller
D
R
D
R
D
R
D
R
Offset
Transmission line: positive
Transmission line: negative
R
S
48
5 I
C
Remove offset resistors on EVBs
located in the middle of the bus .
Term
ina
tion
resistor
Te
rm
ina
tion
re
sistor
R
S
48
5 I
C
R
S
48
5 I
C
R
S
48
5 I
C
CircLink Controller
CircLink Controller
CircLink Controller
Offset
Offset
Offset
