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Manual

7.1.10. RS485 Cable

For best noise immunity use twisted pair cables to make the RS485 connection. In Half Duplex wiring only 1

twisted cable pair is needed. Two twisted pair cables are needed for Full Duplex communications.

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

cable to make the RS485 connection. Terminate the twisted pair cable

7.1.11. RS485 One Talker – Many Listeners, Half Duplex

There are several schemes for connecting RS485 devices depending on the characteristics of the system. In

many cases there will be only one device, which can transmit, data and all the others simply listen to it. This

scheme is used for theatrical lighting intensity control in the DMX512 standard. This is shown below. For the

talker the RS485 TXD GATE jumper should remain in the factory set position, i.e. transmitter is always

enabled. There is NO multiplexing of the TXD and RXD lines. Data is only flowing one way, from PC

outwards, and is thus a Half-Duplex configuration so only one twisted pair cable is needed.

Note: The Receiver end of MAIN line terminated in characteristic impedance by ONBOARD resistor

networks stubs off the main not terminated.

In the above scheme, one RS485 device is talk only, it transmits data, but it does not receive any. The other

RS485 devices are receive only, they do not transmit any data at all.

7.1.12. Half Duplex Settings of RS485 Multiplex

Both jumpers shorted, causing the RXD- & TXD- and the RXD+ & TXD+ signals to short together at the 9-pin

connector.

7.1.13. RS485 Many Talkers – Many Listeners, Half Duplex

Another popular RS485 layout is for multiple talkers and multiple listeners. This is shown below. This is also

known as "party line" transmission. It is imperative to have some method of preventing two devices trying to

drive the data lines at the same time. The normal method is to use the RTS line as a talk enable. The RTS

line should go true immediately prior to the data transmission and go false immediately after the last byte in

the stream is sent.