Product manual

PX Range Product Manual Version 2.3 © Copyright Brainboxes Limited 2013 Page 28 of 45
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, predetermined state and
prevents false start bits and bad data being detected during open circuits.
Fail Safe Short Circuit Detection
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
RS-422 Operation
Generally, in RS-422 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 RS-422
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. In this way, no data will ever be transmitted when the receiver is unable to
accept it, due to a full input buffer etc. And so no data will be lost.
RS-485 Operation
The RS-485 standard is intended for up to 32 driver receiver pairs on the bus. The line drivers used in the
RS-422/485 card are designed to work correctly in both R-S422 and RS-485 systems. The main difference
therefore is in how the system is implemented. Though the card uses a 9 pin D connector, in general, not all
the lines are used for RS-485 systems. The RTS+/- and CTS+/- lines, though driven by the card, are usually
not connected. In two wire, Half-Duplex configurations the TXD+ line is connected to the RXD+ whilst the
TXD- line is connected to the RXD-, only one pair of twisted wire cable is used in RS-485 Half Duplex
communications. The hardware handshaking performed by the CTS+/- and RTS+/- lines in RS-422 systems
are handled by a software protocol in RS-485 systems. In situations where more than one device may
transmit data on the shared data line, each cards RTS line is used as a gating signal to enable the TXD
driver only when that card needs to transmit data. This mechanism prevents bus contention caused by
multiple transmitters holding the line in opposing states. Our cards have a facility which automatically “gates”
the RTS line, thus enabling the transmitter independently of any software. The three wiring schemes given
described below are: -
1. RS-485 One Talker Many Listeners (HALF DUPLEX)
2. RS-485 Many Talkers Many Listeners (HALF DUPLEX.)
3. RS-485 Many Talkers Many Listeners (FULL DUPLEX.)