Terminating Impedance's; There Is No Need To Add Any More At The Pc End - Brainboxes CC-357 User Manual

PCI VelocityRS232/422/485
than one device may transmit data, the RTS line is used as transmit
enable signal, so preventing contention between talkers.
D

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
R
D
Chapter 5 Page 71
Additional RS422/485 Operation
Max Data Rate
12m 10 Mbits/sec
1 Mbits/sec
100 Kbits/sec
R
R