Contemporary Control Systems AI Active Hubs - Manual (Page 19)

TD675100-0M
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to point B requires a paired fiber optic cable and that the light
gray connector at one point must connect to a dark gray
connector at the other point.
2.6.5 Connecting DC Coupled EIA-485 Networks (-485)
A removable 5-position screw connector is provided for each
DC coupled EIA-485 segment. Two connections are provided
for each differential signal phases A and B. The middle pin
connects to a 100 ohm resistor referenced to DC common. This
segment can be up to 900 feet long of IBM type 3 unshielded
twisted-pair cable, and as many as 17 nodes can occupy the
segment. Make sure that the phase integrity of the wiring
remains intact. All phase A signals on the AI network interface
modules and other hubs must be connected together. The same
applies to phase B. Figure 2-6 has been provided to assist in
connecting the various devices. If shielded cable is to be used,
the shielded end can be terminated to the middle pin on the
connector.
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 to the port connector. 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 connect an
external resistor across phase A and phase B.
Failsafe 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 +5V line while