B&B Electronics 3PCIOU4 - Manual - Manual (Page 41)

RS-422/ 485 Connections/Operation
Document Number 3PCI-0712m Chapter 8 35
It is important to maintain the Mark state for all RS-485 receivers when no data is being
sent. Maintaining the line in the Mark state minimizes noise (that would otherwise be
picked up by a high impedance line). It also provides the starting point so that when a
transmitter begins to send data the leading edge of the start bit can transition from the
Mark to Space state.
To maintain the Mark state biasing is required. Biasing is the technique of placing pull
up (to positive voltage) and pull down (to ground) resistors on the line so that all RDA(-)
receiver inputs remain at least 200 mV more negative than RDB(+) inputs (Mark state).
This ensures the inputs will remain above the receiver threshold of 200mV for all
devices on that section of the network. If the biasing is not maintained, the first data bit
of a signal may not be detected, and one or more devices may not respond to
commands, or may operate intermittently.
Figure 18. RS-485 Biasing Resistors
MIport RS-485 receivers come pre-biased from the factory with a 4.7 k pull-up resistor
on the RDB(+) line and a 4.7 k pull-down resistor on the RDA(-) line. These values are
usually adequate for networks that do not implement termination resistors and have 31
or fewer nodes. When termination is used, biasing must be increased (resistors
decreased), calculated according to the termination value and number of nodes.
Through hole pads are provided on MIport cards for adding termination resistors and
bias resistors. The photograph below shows the layout of through hole pads for
installing termination and bias resistors. On all MIport cards the top set of holes is for
the termination resistor. The middle and bottom sets of holes are for bias resistors.
(Note the factory installed 4.7k resistors in the photograph.) Since bias resistors are
always the same value (e.g R pullup = R pulldown = 4.7k) there is no need to identify
which set of holes is for pull up and which is for pull down.