Contemporary Control Systems PCI20U Adapters - Manual (Page 7)

TD030700-0I
G
7
Figure 4 — TB5 NIMs are connected in a daisy-chain fashion.
-485D
DC-Coupled EIA-485 (Non-Backplane Mode)
The PCI20U-485D supports DC-coupled EIA-485 communication via a
daughter board which replaces the coaxial hybrid transceiver.
This daughter
board receives the conventional P1 and P2
pulses intended for the coaxial hybrid
transceiver and converts them to an elongated
P1 pulse (the width is equal to P1 and P2)
suitable for the EIA-485 differential driver.
Therefore, do not set the COM20022 to
backplane mode for EIA-485 communication as
recommended in Standard Microsystems
Corporation’s (SMSC) application note and
data sheet since CC implements the same
signaling on this daughter board.
With our
approach, the same software driver used for
coaxial networks will function with the
EIA-485 version of the PCI20U without modification.
On each NIM, one three-position screw terminal (see Figure 6 ) connector
provides a convenient daisy-chain connection for attaching multiple nodes
onto one segment.
Make sure that the phase integrity of the wiring remains
intact. Be sure to use the proper cable. Refer to Table 3 for connector pin
assignments.
Termination
Each end of the segment must be terminated
in the characteristic impedance
of the cable.
A 120-ohm resistor can be
invoked with a jumper which resides on the
EIA-485 daughter board.
With the middle
jumper inserted at location E1 on the
daughter board, 120 ohms of resistance is
applied across the twisted-pair (
Figure 7 ).
With the jumper removed, no termination is
applied.
If external termination is desired,
remove this jumper and
install a 120-ohm 1/4 watt resistor across pins 1 and 2
on the screw terminal connector.
Incorporating a resistance value less than
Table 2 — Modular Connector
Pin Assignments
Figure 5 — Modular Jack
Numbering Orientation
12345678
120 ohms is not recommended since it may excessively load the EIA-485
transceivers.