7

LTC1346A

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 a balanced twisted pair with 80

Ω

to 120

Ω impedance.

2. The transmitter’s source impedance is between 50

Ω

and 150

Ω.

3. The transmitter’s resistance between shorted termi-

nals and ground is 150

Ω ±15Ω.

4. When terminated by a 100

Ω resistive load, the termi-

nal-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/receiver or

±4V ground potential difference between transmit-

ter and receiver.

Cable Termination

Each end of the cable connected to an LTC1346A must be

terminated by an external Y- or

∆-resistor network for

proper operation. The Y-termination has two series con-

nected 50

Ω resistors and a 125Ω resistor connected

between ground and the center tap of the two 50

Ωresistors

as shown in Figure 6.

The alternative

∆-termination has a 120Ω resistor across

the twisted wires and two 300

Ω resistors between each

wire and ground. 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 better. 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)

Figure 6. Y- and

∆-Termination Networks

50

Ω

50

Ω

125

Ω

Y

LTC1346A • F06

300

Ω

300

Ω

120

Ω

∆