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PCI 1 Port RS422/485
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 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.
Chapter 3 Page 55
Software Setup
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