Datasheet

6
LTC1344A
APPLICATIONS INFORMATION
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Multiprotocol Cable Termination
One of the most difficult problems facing the designer of
a multiprotocol serial interface is how to allow the trans-
mitters and receivers for different electrical standards to
share connector pins. In some cases the transmitters and
receivers for each interface standard can be simply tied
together and the appropriate circuitry enabled. But the
biggest problem still remains: how to switch the various
cable termination required by the different standards.
Traditional implementations have included switching re-
sistors 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. Another method
uses separate termination built on the board, and a custom
cable which routes the signals to the appropriate termina-
tion. Switching the termination using 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.
The LTC1344A solves the cable termination switching
problem via software control. The LTC1344A provides
termination for the V.10 (RS423), V.11 (RS422), V.28
(RS232) and V.35 electrical protocols.
V.10 (RS423) Termination
A typical V.10 unbalanced interface is shown in Figure 4.
A V.10 single-ended generator output A with ground C is
connected to a differential receiver with input 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 5.
In V.10 mode, both switches S1 and S2 are turned off so
the only cable termination is the input impedance of the
V.10 receiver.
Figure 4. Typical V.10 Interface
V.11 (RS422) Termination
A typical V.11 balanced interface is shown in Figure 6. A
V.11 differential generator with outputs A and B with
ground C is connected to a differential receiver with
ground C', inputs A' connected to A, B' connected to B. The
V.11 interface requires a differential termination at the
receiver end that has a minimum value of 100. The
receiver inputs must also be compliant with the imped-
ance curve shown in Figure 7.
In V.11 mode, switch S1 is turned on and S2 is turned off
so the cable is terminated with a 103 impedance.
Figure 5. V.10 Interface Using the LTC1344A
Z
Z
Z
S1
OFF
S2
OFF
124
LTC1344A
V.10
RECEIVER
A
B
C
1344 F05
51.5
51.5
I
Z
–3V
3V 10V
–10V
3.25mA
3.25mA
V
Z
A
CABLE
TERMINATION
LOADGENERATOR
BALANCED
INTERCONNECTING
CABLE
RECEIVER
A
'
CC
'
1344 F04