Brainboxes ES-020 Handbuch PDF herunterladen (Seite 23)

ES Range Product Manual

Version 2.8

Page 23 of 57

RS485 Half Duplex Mode (ES-313, ES-320, ES-346 & ES-842)

For half-duplex operation, the transmit and receive lines of the port need to be connected together

making it a 2 wire system. This can be done by fitting the HD+ and HD- jumpers onto the headers

inside the ES device box..

Half-duplex links usually need biasing resistors fitted at some point on the link. These resistors are

used to hold the signal lines in a known state when the transmitters are off. If these resistors aren‘t

used it is likely that random data will be received when no transmitter is turned on. This is because of

noise which is picked up along the cable which causes the lines to float to unknown states. The B+

resistor pulls the ‗+‘ line up to 3V3 and the B- resistor pulls the other ‗–‗ line down to ground.

It doesn't matter which device on the link has them, as long as one of them does. Biasing resistors

can be connected by fitting the B+ and B- jumpers onto the headers on the ES device PCB.

The T jumper connects in a termination resistor. Termination resistors are used on the end devices in

the link to absorb the signal and stop echoes of the data being reflected back onto the transmission

line.

For point-to-point links (two devices linked), this should be fitted for both devices. For a multi-drop

bus (more than two devices linked on the same cable), the termination should be present at the two

end devices but not at any of the devices in between.

To access the headers, the case of the ES-313 or ES-320 needs to be opened.

Jumpers can be placed vertically across any of the 5 sets of pins explained as thus below:-

B+ and B- = are bias resistors.

T = Termination resistor

HD+ and HD- = are half duplex pins.

So the possible configurations are:

Half-duplex point-to-point link, when no other biasing is present:

B+

3V3

B-

GND

H+

H-

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Brainboxes ES-020 Handbuch Seite 23