ES Industrial Range
Product Manual V3.3
© Copyright Brainboxes Ltd Page 38 of 53
The ES device will have 3 header pins per port which are for electrical
jumpers. These jumpers will control the termination resistors and biasing
resistors. For optimum signal quality, fit the T jumper but not the B+ and B-
jumpers.
The RS-485 standard is similar to the RS-422 standard upon which it is based. The main difference is
that up to 32 transmitter / receiver pairs may be present on the line at one time.
Only one pair of twisted wire cable is used in RS-485 Half Duplex communication, Data+ and Data-.
These are the two main wiring schemes:
RS-485 One Talker Many Listeners (Half Duplex)
RS-485 Many Talkers Many Listeners (Half Duplex)
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 device needs to be opened.
Jumpers can be placed vertically across any of the 3 sets of pins explained as thus below:-
B+ and B- = are bias resistors.
T = Termination resistor
T
B-
GND
B+
3V3
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