6

LTC1344

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 terminations 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 or separate termina-

tions are built on the board, and a custom cable routes the

signals to the appropriate termination. Switching the

terminations 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 LTC1344 solves the cable termination switching prob-

lem via software control. The LTC1344 provides termina-

tion 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 inputs A

nected to A and input B

ground C. The receiver’s ground C

signal return. 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.

A

CABLE

TERMINATION

LOAD

GENERATOR

BALANCED

INTERCONNECTING

CABLE

RECEIVER

A

'

C

B

'

C

'

1344 F04

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

V.11 interface requires a different termination at the re-

ceiver end that has a minimum value of 100

Ω.Thereceiver

inputs must also be compliant with the impedance 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

Ω impedance.

Figure 5. V.10 Interface Using the LTC1344

Z

Z

Z

S2

OFF

124

Ω

LTC1344

V.10

RECEIVER

A

B

C

1344 F05

51.5

Ω

51.5

Ω

–3V

3V

10V

–10V

– 3.25mA

3.25mA

S1

OFF