User`s manual

OP7100 Hardware

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Any Rabbit Semiconductor controller or the OP7100 can be a master or a

slave. A network can have up to 255 slaves, but only one master.

A multidrop network requires termination/bias resistors to minimize reflec-

tions (echoing) and to keep the network line active during an idle state. The

OP7100 termination resistors are already installed, and by default are en-

abled by having jumpers installed on header J9. Remove the jumpers from

header J9, as shown in Figure 3-21, to disable or remove the termination

resistors. Only the first and last devices on a multidrop RS-485 network should

have the termination resistors enabled.

Figure 3-21. Enabling/Disabling Termination Resistors

Only a single, solid conductor should be placed in a screw clamp terminal.

Bare copper, particularly if exposed to the air for a long period before

installation, can become oxidized. The oxide can cause a high-resistance

(~20 Ω) connection, especially if the clamping pressure is not sufficient.

To avoid oxidation, use tinned wires or clean, shiny copper wire. If you are

using multiple conductors or stranded wire, consider soldering the wire

bundle or using a crimp connector to avoid a later loss of contact pressure

to a spontaneous rearrangement of the wire bundle. Note that soldering a

stranded wire may make the wire subject to fatigue failure at the junction

with the solder if there is flexing or vibration.

Termination Resistors

Enabled

Termination Resistors

Disabled

J10