ACL CSC400 Installation And Operation Manual page 40

- Ensure that the "Isolated Ground" terminals are all attached together on all RS485 devices on the bus. This ground
should be connected to earth ground at one point along the bus, preferably at the Master.
- If using a CAT5E (or similar) cable with unused wires, do not leave them "floating". Connect these wires at one point
on the cable to the ground (or "Isolated GND") terminal at the CSC400, or at the master's ground terminal.
- A USB-to-RS485 cable may also have unused wires if the provided Terminator resistor wires are not used (the FTDI
Chip cable as an example). These should be connected to ground as well, to reduce noise propagation.
Termination
An RS-485 bus should only be terminated at each end of the cable (at each device at the end of the cable). No other devices in-
between the two devices at each end should have termination resistors installed or enabled. If there are 20 devices on an RS-
485 bus in a daisy-chain, the 120 ohm termination resistors should only be enabled at the first device and at the 20
The CSC400 Controller has a 4-pin DIP switch with the third switch from the top labeled "120ohm term". This can be used to
connect a built-in 120 ohm resistor. Simply push the third DIP switch to the right and the 120ohm termination resistor will be
connected.
Line Polarization
If Line Polarization is not available on the Master device and is required for the RS-485 bus in this installation, two "Line
Polarization" DIP switches on the CSC400 Controller are available. To enable the Line Polarization terminations, move them
to the right (towards the "Line Pol..." text) as shown in the picture below. If the DIP switches are moved towards the left, the
Line Polarization terminations are removed from the RS-485 bus on this CSC400 device.
"Line Polarization" enables a pullup resistor on the "Data A +" signal and a pulldown resistor on the "Data B –" signal. It
ensures that the bus is put into a known state with the "Data A +" signal High and the "Data B -" signal Low.
Line Polarization should only be enabled on one device on the RS485 bus, if necessary. Usually this is done at the end of the
bus where the master device resides.
Isolated (or Common) Ground
The "Isolated Ground" terminal on each CSC400 Controller is isolated from the onboard CSC400 ground. This isolated ground
connection should be used to connect all common ground connections on all RS-485 devices on the bus. This common ground
should be connected to earth or protective ground at one end of the RS-485 cable only (preferably), usually at the master
device.
Due to the potential for large amounts of noise to be conducted onto the RS485 cable, an option is provided to connect the
RS485 isolated ground to the CSC400 earth ground to shunt noise away locally instead of at the Modbus master. A solid
ground connection should be made between a CSC400 earth ground terminal to an earth ground external to the CSC400 using a
minimum 16AWG wire.
th
device.
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