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LTC1345

Review of CCITT Recommendation V.35

Electrical Specifications

V.35 is a CCITT recommendation for synchronous data

transmission via modems. Appendix 2 of the recommen-

dation describes the electrical specifications which are

summarized below:

1. The interface cable is balanced twisted-pair with 80

Ω to

120

Ω impedance.

2. The transmitter’s source impedance is between 50

Ωand

150

Ω.

3. The transmitter’s resistance between shorted terminals

and ground is 150

Ω ±15Ω.

4. When terminated by a 100

Ω resistive load, the terminal-

to-terminal voltage should be 0.55V

±20%.

5. The transmitter’s rise time should be less than 1% of the

signal pulse or 40ns, whichever is greater.

6. The common-mode voltage at the transmitter output

should not exceed 0.6V.

7. The receiver impedance is 100

Ω ±10Ω.

8. The receiver impedance to ground is 150

Ω ±15Ω.

9. The transmitter or receiver should not be damaged by

connection to earth ground, short-circuiting, or cross

connection to other lines.

10. No data errors should occur with

±2V common-mode

change at either the transmitter or receiver, or

±4V ground

potential difference between transmitter and receiver.

Cable Termination

Each end of the cable connected to an LTC1345 must be

terminated by either one of two electrically equivalent

external Y or

∆ resistor networks for proper operation. The

Y-termination has two series connected 50

Ω resistors and

a 125

Ω resistor connected between ground and the center

tap of the two 50

Ω resistors as shown in Figure 6A.

Figure 6. Y and

∆ Termination Networks

Ω

125

Ω

LTC1345 • F06

300

Ω

300

Ω

120

Ω

The alternative

∆-termination has a 120Ω resistor across

the twisted wires and two 300

Ω resistors between each

wire and ground as shown in Figure 6B. Standard 1/8W,

5% surface mount resistors can be used for the termination

network. To maintain the proper differential output swing,

the resistor tolerance must be 5% or less. A termination

network that combines all the resistors into an SO-14

package is available from:

BI Technologies (Formerly Beckman Industrial)

Resistor Networks

4200 Bonita Place

Fullerton, CA 92635

Phone: (714) 447-2357

FAX: (714) 447-2500

Part #: BI Technologies 627T500/1250 (SOIC)

899TR50/125 (DIP)

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