Termination, Bias – Contemporary Control Systems AI-SRVR Servers to Ethernet Installation Guide User Manual

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TD000200-0IH

Termination

Each end of a segment must be terminated in the characteristic

impedance of the cable. A 120-ohm 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 ¼ watt resistor across pins A and B of the screw

terminal connector. A resistance value under 120 ohms is not

recommended since 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 (Rb) 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-SRVR/485 is at the end

of the segment, install all three jumpers at location E1 on the daughterboard.

Otherwise, remove all three jumpers. To use external termination, removed

the middle jumper at E1.
For EIA-485 DC operation, it is very important that all devices on the

segment be referenced to the same ground potential so that the common

mode voltage requirement (+/–7 Vdc) of the EIA-485 specification is

achieved. This can be accomplished by a separate ground wire tied to all

devices or using the third wire ground of the power connector. Alternately,

connect the DC common of each node to a cold water pipe. Connected

systems having different grounds, can cause unreliable communications

or can damage the drivers. Therefore, adequate grounding must be

implemented. A ground connection can be found at pin 3 of the screw

terminal connector.