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ON
2-Wire
RS-485
OFF
NO Echo
Figure 5 - RS-485, 2-wire without 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. Switches SW1 and SW2 allow the user to customize
this interface to their specific requirements. Each switch position corresponds to
a specific portion of the interface. If multiple ULTRA COMM+I.PCI adapters
are configured in a RS-485 network, only the boards on each end should have
jumpers T, P & P ON. Refer to the following table for each position's operation:
Name
T
Adds or removes the 120 ohm termination.
P
Adds or removes the 1K ohm pull-down resistor in the
RS-422/RS-485 receiver circuit (Receive data only).
P
Adds or removes the 1K ohm pull-up resistor in the RS-422/RS-
485 receiver circuit (Receive data only).
L
Connects the TX+ to RX+ for RS-485 two wire operation.
L
Connects the TX- to RX- for RS-485 two wire operation.
Sealevel Systems ULTRA COMM+I.PCI
1 2 3 4 5 6 7
8 9 10
1 2 3 T P P L L E
Function
Card Setup
Page 3
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