Product manual

Serial Solutions Range Product

Manual

Note: BOTH ends of MAIN line terminated in characteristic impedance, stubs off main line not impedance,

since both ends receive. The twisted pair ends are wired to both RXD+ & TXD+ and RXD- & TXD- at each

RS485 device!

7.1.14. RS485 Many Talkers – Many Listeners, Full Duplex

The RS485 many talkers, many listeners, Full Duplex system can be used when all the RS485 devices have

separate Transmit and Receive channels. There is NO multiplexing of the TXD and RXD signals on the

same device. This system is especially useful when there is no flow control available on the PC, usually due

to the use of a third party communications program that prevents the use of the RTS signal as a "transmit

enable" control, via the TXD GATE jumper. It can be used in the following situations:-

a) The PC is connected to only ONE RS485 device.

b) The PC is communicating with several RS485 devices that are each able to recognize and respond to

their own unique address. The RS485 devices only drive their TXD lines when they are responding to

requests from the PC to send data. In effect, the RS485 device’s address and the command it receives is

used to control access to the devices TXD channel.

This is a Full Duplex system. Two twisted pair cables are required. One twisted pair, is the PC’s TXD

channel, it carries the data sent from the PC’s TXD outputs to the RXD inputs of each of the RS485 devices.

The second twisted pair, is the Devices TXD channel, it carries the data sent from each of the devices’ TXD

outputs to the RXD inputs of the PC.

The advantages of this system are great, since no new communications, software is needed, and the PC can

talk and listen at the same time. In effect, the handshaking is performed by the intelligence of the RS485

devices attached to the PC. When wired as below, the PC can transmit data at any time and all the RS485

devices #1 to #n simultaneously receive it. Only one of the RS485 devices may talk, i.e. transmit data, at any

one time. Each RS485 device recognizes commands and data addressed to it, it only talks when the PC

commands it to do so. When the RS485 device receives the command to talk from the PC, it gates its TXD

drivers on, sends the data down the device TXD channel, and disables its TXD drivers. The other RS485

devices remain in the receive only mode when they are not being addressed, they do not transmit any data

at all.

RS485 Jumpers

Both jumpers left unconnected (not shorted)