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PCI VelocityRS232/422/485
Additional RS422/485 Operation
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
Chapter 5 Page 72
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