Table of Contents
Advertisement
Installation
Special Considerations for High Data Rates
Because of the high data rates possible with the Dual V.34 modem, the type and length of the DTE
cable are important factors in ensuring data integrity. The modem's higher data rates require the use
of shielded DTE cables. Further, the cable's capacitance and resistance, which vary with its length,
must be evaluated to ensure that it can support the chosen data rate over the required distance.
Capacitance and Cable Length
Cable capacitance is specified in two categories: conductor-to-conductor (C-C) and conductor-to-
shield (C-S). Each of those capacitance values is expressed in PicoFarads per foot (pF/ft). To
calculate a cable's total capacitance, add C-C to C-S and multiply the sum by the cable's length in
feet. The following table shows the greatest total capacitance tolerated for each range of data rates,
using an example calculation.
Example Calculation
Example:
A 10-foot cable with a C-C capacitance of 12.5 pF/ft
and a C-S capacitance of 22.0 pF/ft.
Total capacitance = [(C-C) + (C-S)] * 10
Resistance and Cable Length
A DTE cable for use with the modem must have total resistance of 20 ohms or less. To calculate a
cable's resistance, multiply its specified resistance value (in ohms per 1000 feet) by its length (in
feet). The following table shows the greatest lengths that can be used at differing data rates for two
GDC DTE cables: P/N 028H511-XXX (DB25 male-to-female) and P/N 028H502-XXX (DB25
male-to-male), where XXX designates the desired length in feet.
Example
Calculation
Example:
A 10-foot cable with a resistance of
24 ohms/1000 ft.
Total Resistance = (24/1000) * 10
060R122-000
Issue 15
= (12.5 + 22.0) * 10
= 34.5 * 10
= 345 pF
Data Rate
(kbps)
0.0 to 20.0
20.0 to 30.0
30.0 to 60.0
60.0 to 100.0
= 0.024 * 10
100.0 to 128.0
= 0.24 ohms
SpectraComm Dual V.34 Modem
Installation and Operation
Data Rates (kbps)
0.0 to 20.0
20.0 to 30.0
30.0 to 60.0
60.0 to 100.0
100.0 to 128.0
028H511-XXX
-050
-025
-010
-005
-050
X
X
X
X
X
Total Capacitance (pF)
2400
1200
900
450
200
028H502-XXX
-025
-010
-005
X
X
X
X
X
2-7
Advertisement
Table of Contents
