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LTC1543
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APPLICATIO S I FOR ATIO
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Overview
The LTC1543/LTC1544 form the core of a complete soft-
ware-selectable DTE or DCE interface port that supports
the RS232, RS449, EIA530, EIA530-A, V.35, V.36 or X.21
protocols. Cable termination may be implemented using
the LTC1344A software-selectable cable termination chip
or by using existing discrete designs.
A complete DCE-to-DTE interface operating in EIA530
mode is shown in Figure 10. The LTC1543 of each port is
used to generate the clock and data signals. The LTC1544
is used to generate the control signals along with LL (Local
Loopback).The LTC1344A cable termination chip is used
only for the clock and data signals because they must
support V.35 cable termination. The control signals do not
need any external resistors.
Mode Selection
The interface protocol is selected using the mode select
pins M0, M1 and M2 (see the Mode Selection table).
For example, if the port is configured as a V.35 interface,
the mode selection pins should be M2 = 1, M1 = 0, M0 = 0.
For the control signals, the drivers and receivers will
operate in V.28 (RS232) electrical mode. For the clock and
data signals, the drivers and receivers will operate in V.35
electrical mode. The DCE/DTE pin will configure the port
for DCE mode when high, and DTE when low.
The interface protocol may be selected simply by plugging
the appropriate interface cable into the connector. The
mode pins are routed to the connector and are left uncon-
nected (1) or wired to ground (0) in the cable as shown in
Figure 11.
The internal pull-up current sources will ensure a binary 1
when a pin is left unconnected and that the LTC1543/
LTC1544 and the LTC1344A enter the no-cable mode
when the cable is removed. In the no-cable mode the
LTC1543/LTC1544 supply current drops to less than
200µA and all LTC1543/LTC1544 driver outputs and
LTC1344A resistive terminations are forced into a high
impedance state.
The mode selection may also be accomplished by using
jumpers to connect the mode pins to ground or V
CC
.
Cable Termination
Traditional implementations have included switching
resistors with expensive relays, or requiring the user to
change termination modules every time the interface
standard has changed. Custom cables have been used
with the termination in the cable head or separate termina-
tions are built on the board and a custom cable routes the
signals to the appropriate termination. Switching the
terminations with FETs is difficult because the FETs must
remain off even though the signal voltage is beyond the
supply voltage for the FET drivers or the power is off.
Using the LTC1344A along with the LTC1543/LTC1544
solves the cable termination switching problem. Via soft-
ware control, the LTC1344A provides termination for the
V.10 (RS423), V.11 (RS422), V.28 (RS232) and V.35
electrical protocols.
V.10 (RS423) Interface
A typical V.10 unbalanced interface is shown in Figure 12.
A V.10 single-ended generator output A with ground C is
connected to a differential receiver with inputs A' con-
nected to A, and input C' connected to the signal return
ground C. Usually, no cable termination is required for
V.10 interfaces, but the receiver inputs must be compliant
with the impedance curve shown in Figure 13.
The V.10 receiver configuration in the LTC1544 is shown
in Figure 14. In V.10 mode switch S3 inside the LTC1544
is turned off.The noninverting input is disconnected inside
the LTC1544 receiver and connected to ground. The cable
termination is then the 30k input impedance to ground of
the LTC1544 V.10 receiver.
