Rabbit OP7100, Руководство пользователя

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OP7100 Hardware 41
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.
FD
Termination Resistors
“Enabled”
Termination Resistors
“Disabled”
J10
J9
1
2
3
4
J9
1
2
3
4
J9