Terminating Impedance's - Brainboxes CC-324 User Manual

PCI 1 Port RS422/485
more than one device may transmit data, the RTS line is used as
transmit enable signal, so preventing contention between talkers.


Terminating Impedance's.

RS422 and RS485 lines should be terminated at the end of
the main branch of the RECEIVER, in the cables characteristic
impedance. These terminating impedance's stop echoes caused by
the serial data being reflected back at the cable ends. It is not
necessary to terminate the transmitter end of the twisted pair.
The AT Dual Port RS422/485, Opto Isolated AT Dual Port
RS422/485 and AT Velocity RS422/485 cards have the correct
120 Ohm (nominal) terminating resistors for the RXD twisted pair
line and the CTS twisted pair line fitted on the RS422/485 card for
both the serial ports on the card.
There is no need to add any more at the PC end.
The terminating impedance's shown later in the wiring
diagrams of Figure 4-1, Figure 4-2, Figure 4-4 & Figure 4-6 are
automatically provided by the on board resistors and do not have
to be added by the user.
RS485 Standard
Up to 32 Driver/Receiver Pairs
Line Length
40 Feet =
400 Feet = 122m
4000 Feet = 1219m
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Chapter 3 Page 54
Max Data Rate
12m 10 Mbits/sec
1 Mbits/sec
100 Kbits/sec
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Software Setup
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