13
LTC2845
sn2845 2845fs
APPLICATIONS INFORMATION
WU
U
Figure 16. Typical V.28 Interface
AA
'
CC
'
GENERATOR
BALANCED
INTERCONNECTING
CABLE
LOAD
CABLE
TERMINATION
RECEIVER
2845 F16
Figure 17. V.28 Receiver Configuration
R5
20k
LTC2845
RECEIVER
2845 F17
R8
6k
S3
R6
10k
R7
10k
R4
20k
A
'
B
'
C
'
GND
V.28 (RS232) Interface
A typical V.28 unbalanced interface is shown in Figure 16.
A V.28 single-ended generator output A with ground C is
connected to a single-ended receiver with input A
' con-
nected to A, ground C
' connected via the signal return
ground C.
In V.28 mode, all switches are off except S3 inside the
LTC2846/LTC2845 which connects a 6k (R8) impedance
to ground in parallel with 20k (R5) plus 10k (R6) for a
combined impedance of 5k as shown in Figure 17. The
noninverting input is disconnected inside the LTC2846/
LTC2845 receiver and connected to a TTL level reference
voltage for a 1.4V receiver trip point.
V.35 Interface
A typical V.35 balanced interface is shown in Figure 18. A
V.35 differential generator with outputs A and B with
Figure 19. V.35 Receiver Configuration
Figure 18. Typical V.35 Interface
A
'
B
C
B
'
C
'
GENERATOR
BALANCED
INTERCONNECTING
CABLE
LOAD
CABLE
TERMINATION
RECEIVER
2845 F18
50
125
50
50
125
50
R3
124
R5
20k
LTC2846
RECEIVER
2845 F19
A
'
B
'
C
'
R1
51.5
R8
6k
S2
S3
R2
51.5
R6
10k
R7
10k
GND
R4
20k
S1
ground C is connected to a differential receiver with
ground C
', inputs A' connected to A, B' connected to B. The
V.35 interface requires a T or delta network termination at
the receiver end and the generator end. The receiver
differential impedance measured at the connector must be
100
±10, and the impedance between shorted termi-
nals (A
' and B') and ground C' must be 150
±15.
In V.35 mode, both switches S1 and S2 inside the LTC2846
are on, connecting the T network impedance as shown in
Figure 19. The 30k input impedance of the receiver is
placed in parallel with the T network termination, but does
not affect the overall input impedance significantly.
The generator differential impedance must be 50
to
150
and the impedance between shorted terminals (A
and B) and ground C must be 150
±15. For the
generator termination, switches S1 and S2 are both on as
shown in Figure 20.