The RS485 standard imposes a differential level of at least 200 mV to detect the signal level.
To do that, the polarisation resistors must be at one end of the bus, usually at the master level.
A simple method to check the correct polarisation consists in positioning the polarisation
source at the start of the bus (master side) and to check the voltage level at the other end of
the bus.
The common terminal (ground) must be interconnected with the corresponding terminals on
each appliance to make sure the voltage between them is balanced. If the common conductor
is not installed between all the devices, they must be properly grounded in compliance with
the manufacturer recommendations for each network device. This requirement implies the
use of an extra wire which, although not part of the communication process, is essential to
guarantee the electric integrity of the network devices.
For more information on the RS4875/RS422 standards and device wiring, refer to
the EIA-485 and EIA RS-422-A standards.
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