Contemporary Control Systems EXP Expansion Modules User Manual - Manual (Page 16)

to the RJ-11 connector on the expansion module. With the middle
jumper inserted at location E1 on the daughterboard, 120 ohms of
resistance is applied across the twisted-pair.
With the jumper
removed, no termination is applied.
If it is desired to apply
external termination instead, remove this jumper and insert an
RJ-11 style tap (not provided) and terminator in the RJ-11 port.
Bias
In addition to the termination, it is also necessary to apply bias
to the twisted-pair network so that when the line is floated
differential receivers will not assume an invalid logic state.
There are two precision bias resistors (Rb) of equal value on
each daughterboard. One resistor is tied to the +5 V line while
the other is tied to ground. Each resistor has a jumper associated
with it. If the two jumpers are installed, the resistor tied to +5 V
is connected to the (+) signal line while the grounded resistor is
connected to the (–) line. This voltage drop will bias the
differential receivers into a defined state when no differential
drivers are enabled. Differential receivers typically switch at or
near zero volts differential and are guaranteed to switch at
+/–200
mV. Through the transition point, 70 mV of hysteresis
will be experienced. Therefore, a positive bias of 200
mV or
greater will ensure a defined state. We recommend that bias be
applied to both ends of the wiring segment by installing the two
end jumpers located at position E1 on the daughterboard. This
is to be done for only the hub ports or NIMs located at the ends
of the segment. All other NIMs will have their jumpers removed.
The termination and bias rules are simple. If the NIM or hub
port is located at the extreme ends of the segment, install all
three jumpers at location E1 on the daughterboard. If the NIM
is located between the two end NIMs or hub ports, remove all
three jumpers. If external termination is desired, remove the
middle jumper at E1 and provide the external termination.
TD401600-0M
O
1
2
Termination
Each end of the segment must be terminated in the characteristic
impedance of the cable.
A 120-ohm resistor can be invoked with
a jumper which resides on the EIA-485 daughterboard adjacent