12
LTC2844
sn2844 2844fs
Cable Termination
Traditional implementations have included switching
resistors with expensive relays, or required 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
termination 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 LTC2846/LTC2844 solves the cable termination
switching problem. Via software control, appropriate ter-
mination for the V.10 (RS423), V.11 (RS422), V.28 (RS232)
and V.35 electrical protocols is chosen.
V.10 (RS423) Interface
A typical V.10 unbalanced interface is shown in Figure 11.
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 12.
The V.10 receiver configuration in the LTC2844 is shown
in Figure 13. In V.10 mode switch S3 inside the LTC2844
is turned off.The noninverting input is disconnected inside
the LTC2844 receiver and connected to ground. The cable
termination is then the 30k input impedance to ground of
the LTC2844 V.10 receiver.
I
Z
V
Z
–10V
–3.25mA
3.25mA
–3V
3V 10V
2844 F12
Figure 12. V.10 Receiver Input Impedance
Figure 11. Typical V.10 Interface
Figure 13. V.10 Receiver Configuration
AA
'
CC
'
GENERATOR
BALANCED
INTERCONNECTING
CABLE
LOAD
CABLE
TERMINATION
RECEIVER
2844 F11
R5
20k
LTC2844
RECEIVER
2844 F13
A
'
B
'
C
'
R8
6k
S3
R6
10k
R7
10k
GND
R4
20k
APPLICATIONS INFORMATION
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