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Cisco IGX 8400 Series Switches

V.35/RS449 Cable Length Recommendations

Cisco - V.35/RS449 Cable Length Recommendations

Document ID: 10795

Updated: Apr 17, 2009

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Introduction

This document describes the recommended maximum cable length values for normal timing modes and loop timing modes.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

For more information on document conventions, refer to the Cisco Technical Tips Conventions.

Recommended Maximum Cable Length Values

These cable length values are the maximum recommended for normal timing modes:

V.35/RS449 Data Rate Max cable length recommended (feet) Max cable length recommended (meters)
2 Mb/sec 50 15.24
1 Mb/sec 100 30.48
512 Kb/sec 200 60.96
256 Kb/sec 400 121.92
128 Kb/sec 800 243.84
56 K 1600 487.68
1.2 Kb/sec 3000 914.40

This assumes the cable has these characteristics:

  • 24 AWG

  • 100 Ohm

  • Copper twisted pair cable with maximum capacitance of 16 pf/foot (standard telephone cable)

The EIA422 standard specifies a graph of cable length versus data rate based on AC attenuation. The real limiting factor is clock margin when the timing is set to normal. If the timing is set to loop, the graph applies instead.

Clock margin is calculated by assuming a maximum round trip delay of 4 ns/ft of the cable, with respect to half the period of the clock plus some guardband.

For loop timing mode, use this table (extracted from graph). These cable length values are the maximum recommended for loop timing modes:

V.35/RS449 Data Rate Max cable length recommended (feet) Max cable length recommended (meters)
2 Mb/sec 150 45.72
1 Mb/sec 300 91.44
512 Kb/sec 600 182.88
256 Kb/sec 1200 365.76
128 Kb/sec 2400 731.52
56 K 3000 914.40
1.2 Kb/sec 3000 914.40

Related Information

Updated: Apr 17, 2009
Document ID: 10795