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© Sealevel Systems, Inc. 7402e Manual | SL9251 11/2022
Electrical Interface Selection
Each of the four ports on the COMM+4.422.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
Line Termination
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+4.422.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.
RS-485 ‘Echo’
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.