Contemporary Control Systems PCI20U Adapters - Manual (Page 8)

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 daughter board.
One resistor is tied to the +5 V
line and 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 the “1” 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 daughter board.
This is to be done for only the two NIMs located at the end of the segment.
All other NIMs will have their jumpers removed.
The termination and bias rules are simple.
If the NIM is located at the
extreme ends of the segment, install all three jumpers at location E1 on the
daughter board.
If the NIM is located between the two end NIMs, remove all
three jumpers.
If external
termination is desired,
remove
the middle jumper
at E1.
For EIA-485 DC operation, it
is very important that all
devices on the wiring segment
be referenced to the same
ground potential in order that
the common mode voltage requirement (+/–7 Vdc) of the EIA-485
specification is achieved.
This can be accomplished by running a separate
ground wire between all computers or by relying upon the third wire ground
of the power connector assuming that the DC power return is connected to
chassis ground on the computer.
Another approach would be to connect the
DC common of each computer to a cold water pipe. Connected systems, each
with different elevated grounds, can cause unreliable communications or
can
damage the EIA-485 differential drivers. Therefore, it is important that an
adequate grounding method be implemented. A ground connection can be
found at pin 3 of the screw terminal connector.
TD030700-0I
G
8
Table 3 — Screw Terminal Connector Pin
Assignments for -485, -485D and -485X