American Fibertek MRX-8685C - Manual (Page 11)

11
OFFSET BIAS – RS485
The RS485 specification requires receivers to detect input signals down to 200mVp-p of voltage
level. In many cases this can cause systems to be sensitive to noise on the data wires. In an
effort to eliminate the effects of low levels of noise, some manufacturers of equipment that
communicate using RS485 have introduced a small voltage bias to the data lines. This is
usually accomplished using a 470 Ohm resistance to +5V on the positive line and 470 Ohm
resistance to ground on the negative line. When used in conjunction with the appropriate
termination resistors referred to in the previous section, this introduces about a 300 mV offset,
improving noise immunity.
Offset bias for four wire RS485 can be applied on the MTX-8685C and MRX-8685C by setting
switch # 1 and switch # 3 down. Please note that these switches work as a set and must be
either both down or both up. In a majority of cases, bias will not be required and these switches
should remain in the up position. If using offset bias, it is important that termination resistor
switch # 2 and switch # 5 be in the down (applied) position. Using offset bias without a
termination on the line will cause communications to fail.
Offset bias for two wire RS485 can be applied on the MTX-8685C and MRX-8685C by setting
switch # 4 and switch # 6 down. Please note that these switches work as a set and must be
either both down or both up. In a majority of cases, bias will not be required and these switches
should remain in the up position. If using offset bias, it is important that termination resistor
switch # 5 be in the down (applied) position. Using offset bias without a termination on the line
will cause communications to fail.