6

LTC1344A

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

nected to A and input C

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

V.11 interface requires a differential termination at the

receiver end that has a minimum value of 100

Ω. 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

Ω impedance.

Figure 5. V.10 Interface Using the LTC1344A

Z

Z

Z

S1

OFF

S2

OFF

124

Ω

LTC1344A

V.10

RECEIVER

A

B

C

1344 F05

51.5

Ω

51.5

Ω

–3V

3V

10V

–10V

– 3.25mA

3.25mA

A

CABLE

TERMINATION

LOAD

GENERATOR

BALANCED

INTERCONNECTING

CABLE

RECEIVER

A

'

CC

'

1344 F04