Contemporary Control Systems AI-USB Adapters - Manual (Page 9)

TD030500-0IB
9
485 DC-Coupled EIA-485
Termination
Each end of the 485 segment must be terminated in the
characteristic impedance of the cable. An internal resistance
can be invoked with a jumper on the EIA-485 daughter board.
With the middle jumper inserted at E1 on the daughter board,
120 ohms of resistance is applied across the twisted-pair. With
the jumper removed, no termination is applied. To apply
external termination instead, remove this jumper and install a
120-ohm 1/4 watt resistor across pins A and B of the screw
terminal connector. A value under 120 ohms is not recommended
because it may excessively load the EIA-485 transceivers.
Bias
In addition to termination, bias must be applied to the ends of a
twisted-pair network (and nowhere else) so that when the line is
floated, differential receivers will not assume an invalid logic
state. There are two precision bias resistors of equal value on
the daughter board, one tied to +5 V and the other to ground.
Each resistor has a jumper associated with it. If both jumpers
are installed, the resistor tied to +5 V is connected to the (+)
signal and the grounded resistor is connected to the (–) line.
The termination and bias rules are simple: If the AI-USB is at
the segment end, install all three jumpers at location E1 on the
daughter board (
Figure 3 ). Otherwise, remove all jumpers.
To use external termination, remove the middle jumper at E1.
A removable 5-position screw connector ( Figure 1 ) is provided
for each DC-coupled EIA-485 segment. Two terminals are
provided for differential signal phase A and two for phase B.
The middle pin connects directly to chassis (the enclosure). A
DC-coupled segment can be up to 900 feet of IBM type 3 (or
better) STP or UTP cable, and as many as 17 nodes can occupy
a segment. Make sure the phase integrity of the wiring remains
intact. All phase A signals on the network interface modules
and hubs must be connected together. The same applies to
phase B. If shielded cable is used, the shield should tie to the
middle pin on the connector. Be sure the data rate is the same
for all network devices (see
Figure 5 and associated explanation).