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Initial Setup
37
4.7.3
Using RS-485 LAN to Daisy-Chain
If several 8742 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 8742 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 8742 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 8742 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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