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4.7.3
Using RS-485 LAN to Daisy-Chain
If several 8743-CL units need to be controlled via Ethernet or USB, they do not all have to
be connected directly to an Ethernet Switch or USB Hub. Instead, a single unit can act as
a gateway by connecting it to the Router, Switch, or Hub and also connecting it to the
remaining units using a half-duplex RS-485 LAN bus. In these configurations, the
"Gateway" unit, henceforth referred to as the Master Controller, will be a bus master while
the other units will be slaves.
To facilitate these connections, each 8743-CL has two 3.5mm phone jacks connected in
parallel. The wiring convention is TIP is D+, RING is D-, and SLEEVE is ground.
The normal convention with RS-485 is a daisy-chain configuration of devices; a "STAR"
configuration is not supported. A nominally 100-ohm resistance is applied between the
D+ and D- wires at each end of the bus, but not at the units in the middle of the bus. To
simplify implementation of these bus termination resistances, the 8743-CL applies a 100-
ohm termination between the D+ and D- wires UNLESS a plug is inserted into BOTH of
the 3.5mm jacks. In other words, the two 3.5mm jacks in the 8743-CL each have
integrated switches that go open-circuit when a plug is inserted into the connector. The
switches in the two jacks are connected in parallel, as are the TIP, RING, and SLEEVE
connections. Thus, the termination will be applied between the D+ and D- wires at each
end of the daisy-chain. Units in the middle of the daisy-chain will not apply their
termination resistances because both of their jacks will be occupied.
Connecting four units to a computer via RS-485 LAN
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