Specifications

Diagrams are for reference only.

Consult the appropriate installation sheet for wiring details.

REFERENCE SECTION

NETWORK DATA RISER LIMITSNETWORK DATA RISER LIMITS

NETWORK DATA RISER LIMITS

Resistance is generally specified in ohms per 1,000 feet, and must be doubled for 1,000

feet of a twisted-pair cable. Capacitance is specified in picofarads per foot (pF/ft). The

capacitance between conductors of a twisted-pair is commonly referred to as

conductor-

conductor

mutual

capacitance. Shielded cable has an additional capacitance between

each conductor and the shield. The capacitance of either conductor to shield is typically

twice the value of mutual capacitance, and the highest value of capacitance must be used

when calculating the maximum length of a data network. The overall length of data

networks designed with twisted-pair cable is about twice as long as data networks

designed with shielded cable due to the additional capacitance resulting from the shield.

Calculating a maximum length

The maximum length of a data network can be calculated by dividing the maximum

cumulative capacitance allowed by the highest capacitance rating of the selected cable.

For example, say you wanted to determine maximum length of a data network using

18 AWG cable that is rated at 25 pF per foot. The network will communicate at 38.4

Kbaud. The maximum length equals the maximum cumulative capacitance divided by

the capacitance per foot. In equation form:

ML = MCC / CPF

In our example: ML = 1.4 mF / 25 pF/ft; ML = 56,000 ft

Calculating maximum wire capacitance per foot

The capacitive property of twisted-pair cable varies and the cost of cable generally

increases as the capacitance per foot decreases. Following is a sample calculation for

determining the maximum capacitance per foot that a cable can have for a given

network length. The maximum capacitance per foot equals the maximum cumulative

capacitance divided by the total network length. In equation form:

MCPF = MCC / TNL

Where: • MCC can be obtained from the table given in this topic

• Total network length is the sum of the lengths of individual copper

runs in the network

Example: The total copper distance of a network is 26,000 ft. Calculate the maximum

capacitance per foot that can be used for 18 AWG twisted-pair cable at 38.4K baud.

MCPF = MCC / TNL

MCPF = 1.4 mF / 26,000 ft

MCPF = 53.8 pF/ft