Product manual

Serial Solutions Range Product

Manual

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 are automatically provided by the on board

resistors and do not have to be added by the user.

7.1.4. 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.

7.1.5. 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.

7.1.6. 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. 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.