PCMCIA 1 Port RS422/485
Additional 422/485 Operation
Chapter 4 Page 36
transmit enable signal, so preventing contention between talkers.
RS485 Standard
Up to 32 Driver/Receiver Pairs
Line Length
Max Data Rate
40 Feet = 12m
10 Mbits/sec
400 Feet = 122m
1 Mbits/sec
4000 Feet = 1219m
100 Kbits/sec
D
D
R
R
R
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.
Fail Safe Open Circuit Detection
.
Open circuit is when there are no drivers on the circuit. This
occurs by design in party line multi driver/receiver systems and
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