PCMCIA 1 Port RS422/485
Additional 422/485 Operation
Chapter 4 Page 37
unintentionally when the twisted pair line is accidentally cut or
disconnected or the transmitting device fails. In RS485 party line
systems there are extended periods of time when none of the many
possible talkers are gated onto the bus. This is known as the line idle
state and occurs when all the driver outputs are in the high
impedance state. The lines float, perhaps being pulled to the high or
low state by noise or other voltages on the line.
Without fail safe open circuit detection false start bits are
detected by the receivers, either corrupting good communications or
causing noise to masquerade as good data.
The on board fail safe open circuit detection causes the
receiver to go to a known, pre-determined state and prevents false
start bits and bad data being detected during open circuits.
Fail Safe Short Circuit Protection.
Short circuits are when the two lines of a twisted pair are
connected together. This occurs due to either accidental damage to
the cable or due to failure of one or more transmitter/receivers on the
line. The short circuit condition is dangerous since damage to the
receiver may occur and communication may be corrupted or
prevented.
The on board fail safe short circuit detection prevents the line
impedance from going to zero and thus protects the inputs of
receivers and the outputs of drivers.
RS422 Operation.
Generally, in RS422 systems all 8 signal lines from the 9 pin
D connector participate in the data transfer sequence, thus 4 twisted
pair cables are used. One twisted pair carries the TXD data outwards,
one pair brings the RXD data inward, another pair carries the RTS
handshake outwards and the fourth pair brings the CTS handshake
inwards. There is no need to carry the ground from one device to
another.
This RS422 arrangement allows data to be transmitted and
received simultaneously since each signal has its own data cable
pair. In addition, the receiver can set RTS true so telling the
transmitter on its CTS input that the receiver is ready to accept data.
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