LTC2845 datasheet(11/20 Pages) LINER

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LTC2845

sn2845 2845fs

APPLICATIONS INFORMATION

Mode Selection

The interface protocol is selected using the mode select

pins M0, M1 and M2 (see the Mode Selection table).

For example, if the port is configured as a V.35 interface,

the mode selection pins should be M2 = 1, M1 = 0, M0 = 0.

For the control signals, the drivers and receivers will

operate in V.28 (RS232) electrical mode. For the clock and

data signals, the drivers and receivers will operate in V.35

electrical mode. The DCE/DTE pin will configure the port

for DCE mode when high, and DTE when low.

The interface protocol may be selected simply by plugging

the appropriate interface cable into the connector. The

mode pins are routed to the connector and are left uncon-

nected (1) or wired to ground (0) in the cable as shown in

Figure 10.

The internal pull-up current sources will ensure a binary 1

when a pin is left unconnected and that the LTC2846/

LTC2845 enters the no-cable mode when the cable is

removed. In the no-cable mode the LTC2846/LTC2845

supply current drops to less than 1000

µA and all driver

outputs are forced into a high impedance state.

The mode selection may also be accomplished by using

jumpers to connect the mode pins to ground or VIN.

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/LTC2845 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.

Figure 10. Single Port DCE V.35 Mode Selection in the Cable

VIN

CABLE

3.3k

2845 F10

LTC2846

LTC2845

CONNECTOR

(DATA)

DCE/DTE

D4ENB

R4EN

(DATA)

LTC2845 Datasheet(PDF) 11 Page - Linear Technology

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