B&B Electronics 3PCIU4 - Manual (Page 69)

RS-422/ 485 Connections/Operation
Manual Documentation Number 3PCIoUx-1008 Chapter 8 63
The RS-485 mode is set by configuring the DIP switch setting on the MIport
card and by selecting RS-485 Mode under RTS Control in the Advanced
COM port settings dialog box.
Note: For more information on COM port settings refer to Chapter 2
and Chapter 5
RS-485 Termination Resistors
In some applications termination resistors must be connected across the
communications line to ensure reliable communications. Termination
resistors absorb signal reflections that can occur when a data signal reaches
the end of a transmission line and encounters an impedance higher than that
of the transmission line itself. Typically termination resistors are not required
for communications links that operate at less that 19.2 kbps. At higher bit
rates a termination resistor of 120 ohms may be required. Termination
resistors should be connected across the communications line at the extreme
opposite ends of the network.
Through hole pads are provided on MIport cards to install termination
resistors. When adding termination resistors the value of biasing resistors
must be appropriately sized as well.
RS-485 Network Biasing
Unlike RS-422, where the transmitter holds the TD(B)+ line high and the
TD(A)- line low (in the idle, or Mark state) when not transmitting data, in a
RS-485 network, the transmitter tri-states to a high impedance state when not
sending data.
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 RD(A)- receiver inputs remain at least 200 mV more
negative than RD(B)+ 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.