Product manual

Serial Solutions Range Product

Manual

7.1.7. RS422 Serial Port Cables

Use screened twisted pair Belden cable 9729 and 9829, L type 2493 and 2919 or IBM Part No 4716748

cable to make the RS422 connection. Unscreened Belden type 8795 may also be used in less noisy

environments.

The on board resistor networks terminate the receiving end of the twisted pair cable in its characteristic

impedance.

7.1.8. Serial Port 1 to Other PC Cable

7.1.9. RS485 Operation

The RS485 standard is intended for up to 32 driver receiver pairs on the bus. The line drivers used in the

Serial Solutions RS422/485 card are designed to work correctly in both RS422 and RS485 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 RS485 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 RS485

Half Duplex communications.

The hardware handshaking performed by the CTS+/- and RTS+/- lines in RS422 systems are handled by a

software protocol in RS485 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, i.e. set TXD GATE or AUTO jumper. This mechanism prevents bus contention

caused by multiple transmitters holding the line in opposing states. Revision 3 and higher versions of this

card have a facility which automatically “gates” the RTS line, thus enabling the transmitter independently of

any software. This “Auto gating” is described in more detail in the previous 485 half duplex section

The three wiring schemes given described below are: -

RS485 One Talker Many Listeners (HALF DUPLEX)

RS485 Many Talkers Many Listeners (HALF DUPLEX.)

RS485 Many Talkers Many Listeners (FULL DUPLEX.)