©Sealevel Systems, Inc.
SL9249 – 12/2013
7802e Manual
8
Each of the eight ports on the COMM+8.422.PCIe can be individually configured as an RS-422,
or RS-485 interface. This is selectable via the port DIP-switch, each is labeled with its port
number.
Switch 1 (Silk M1) Switch 2 (Silk M0)
Mode Select
OFF
OFF
RESERVED
OFF
ON
422
ON
OFF
485 With Echo
ON
ON
485 No Echo
Typically, each end of the RS-485 bus must have line-terminating resistors (RS-422 terminates
at the receive end only). A 120-ohm resistor is across each RS-422/485 input in addition to a
1K-ohm pull-up/pull-down combination that biases the receiver inputs. Each switch allows
customization of this interface to specific requirements. Each switch position corresponds to a
specific portion of the interface. If multiple COMM+8.422.PCIe adapters are configured in an RS-
485 network, only the boards on each end should have switches T, P & P ON. Refer to the
following table for each position’s operation:
Switch
Name
Function
3
T
Adds or removes the 120 ohm termination.
4
PU
Adds or removes the 1K ohm pull-up resistor in the RS-422/RS-485 receiver circuit
5
PD
Adds or removes the 1K ohm pull-down resistor in the RS-422/RS-485 receiver circuit
6
L
Connects the TX- to RX- for RS-485 two-wire operation.
7
L
Connects the TX+ to RX+ for RS-485 two-wire operation.
In ‘RS-485 With Echo’ mode both the receiver and the transmitter are enabled simultaneously.
Every time a character is transmitted; it is also received. This can be beneficial if the software
can handle echoing (i.e. using received characters to throttle the transmitter) or it can confuse
the system if the software does not. An RS-485 ‘No Echo’ option is selected by placing both
Mode switches (M0, M1) in the ‘On’ position.
