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Cable Length Limits

Serial Cable Length Limits (RS232, V.35,  RS530, RS422/449)
SCSI Cable Length Limits
Parallel Cable Length Limits


Serial Cable Length Limits (RS-232, V.35,  RS-530, RS-422/449)

Table below lists the maximum permissible cable lengths for various protocols  at various  representative data rates. These lengths are as specified in the EIA RS232C, CCITT V.55 and RS422 interface standards.

Material requirements:

      * Conductor size and type: 24 AWG, twisted pair.
      * Nominal capacitance: 15.5 picofarads per foot (between
conductors of each pair);
        27.5 picofarads per foot (from each conductor to shield).
      * Nominal DC resistance (each conductor): 24.0 ohms per 1000 feet.



      * Nominal DC resistance (shield): 2.35 ohms per 1000 feet.
      * Shield type: Overall braided with drain wire.
      * Approval: Underwriters Laboratories (UL).
     

DTE Data Rate [kb/s]

Maximum DTE Cable Lengths [ft]

RS232C

V.35

RS449/422

RS530/422

56 or 64

10

4000

4000

4000

224 or 256

N/A

3500

1700

1700

896 or 1024

N/A

1700

350

350

N/A - not aplicable (these rates cannot be used for RS232C interface


NOTE on RS-232 Serial Cables

At lower peripheral speeds of up to 9600 baud,serial interfaces can usually operate over cables up to 50 feet long. Use of special high quality cable may in some cases allow this limit to be extended to as much as 150 feet.


SCSI Cable Length Limits

In a SCSI environment, devices are daisy-chained together using a common cable. Both ends of the cable are terminated. All signals are common between all SCSI devices.

There are two types of SCSI devices:  Single-Ended and Differential On differential devices all signals consist of two lines denoted +SIGNAL and -SIGNAL, while for Single-Ended devices all signals consist of one line (SIGNAL).Single-Ended and Differential devices CAN NOT be mixed on the same SCSI cable!

Single-Ended Cable Length Limits:

The maximum cable length shall be 6 meters (primarily for connection within a cabinet)   A stub length of no more than 0.1 meters is allowed.

Differential Cables Length Limits

The maximum cable length shall be 25 meters (primarily for connection outside of a cabinet). A stub length of no more than 0.2 meters is allowed.


Impedance Requirements

IDEAL characteristic impedance:
     - Single-Ended cables: 132 ohms
     - Differential cables: 122 ohms

However, cables with such high characteristic impedance are not usually available. As a result, the SCSI standard requires the following:

  * For unshielded flat or twisted-pair cables: 100 ohms +/-10%
  * For shielded cables                       : 90 ohms or greater

A somewhat lower characteristic impedance is acceptable since few cable types meet the above requirements. Trade-offs in shielding effectiveness, characteristic impedance, cable length, number of loads, transfer rates, and cost, can be made to some limited degree.

NOTE:
To minimize discontinuities and signal reflections, cables of different impedances should not be used in the same bus.


Parallel Cables Length Limits

Older style Centronics parallel cables,  can rarely be more than 15 feet long and still operate correctly, and 9 to 12 feet is a safer limit. The IEEE-1284 can go up to 30feet in length.