Note
You do not have to add a terminator resistor to your cables when you use the PCI-422/485-2
card. Termination resistors for the RX+ and RX- lines are provided on the card and are placed
in the circuit when you install the termination jumpers. Moreover, installing the BIAS jumpers
properly biases these lines. (See the Option Selection section of this manual.)
RS485 Data Transmission
The RS485 Standard allows a balanced transmission line to be shared in a party-line mode. As
many as 32 driver/receiver pairs can share a two-wire party line network. Many characteristics
of the drivers and receivers are the same as in the RS422 Standard. One difference is that the
common mode voltage limit is extended and is +12V to -7V. Since any driver can be
disconnected (or tristated) from the line, it must withstand this common mode voltage range
while in the tristate condition.
RS485 Two-Wire Multidrop Network
The following illustration shows a typical multidrop or party line network. Note that the
transmission line is terminated on both ends of the line but not at drop points in the middle of
the line.
Figure A-1:
Typical RS485 Two-Wire Multidrop Network
