Contemporary Control Systems BASgatewayLX - Manual (Page 11)

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11
EIA-485 Bias and Termination Jumpers
After removing the gateway cover, three jumpers are accessible on a 6-pin jumper block
located near the EIA-485 connector. With all jumpers installed (default condition), this
resistor network imparts to the bus approximately 245 mV of bias via the upper jumper
(pull-up) and the lower jumper (pull-down), and 130 Ω of termination impedance with
the middle jumper. With all three jumpers in place, the effective termination resistance
is 120 Ω — consistent with the EIA-485 standard. It is recommended that the gateway
be installed at the end of a segment — but if installed mid-span, remove all the jumpers
and install 120 Ω resistors at each far end of the bus. If no bias source is available for
the bus, apply the gateway’s internal bias by installing the outer jumpers while leaving
the inner jumper location vacant.
Figure 11 — Internal Termination and Bias