0049-1800-xxx 326X V.34, V.34-SDC and V.32bis Series Modem – USER’S GUIDE 02/23/2010
Rev. A
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Cable Considerations
Raymar-Telenetics has specified a maximum allowable cable capacitance for use with the expanded
data rates of 326X modems. You must determine the correct capacitance of the cable you intend to
use with the modem.
Table B-4 shows a range of data rates and maximum allowable capacitances. The maximum
capacitance is a total of the inter-conductor capacitance (between the conductors represented as Cc in
Table B-4), and the capacitance between the shield and the conductor (represented as Cs in Table B-
4.)
The maximum allowable cable capacitance decreases as DTE rates increase. The more capacitance in
the cable, the shorter the allowable length at a DTE rate. Similarly, the allowable lengths become
shorter as DTE rates increase.
Since newer PCs support EIA 232-D type receivers/drivers at higher rates, you must use information
in this section to determine cable length.
To determine the per foot capacitance of your cable:
1)
Determine the length of the cable you require for your application.
2)
Determine the range of rates that the modem will use. (For example, to routinely operate the
modem’s DTE port between 20.0 and 96.0 kbps, determine the cable capacitance for the higher
rate. You can operate the modem up to, but not exceeding, the maximum rate capacitance that
you calculate.)
3)
Refer to Table B-4 for the maximum allowable cable capacitance for a given rate. (For
example, to operate the modem at 64.0 kbps, the maximum capacitance is 900 pF.)
4)
Divide the maximum cable capacitance by the length of the cable. This results in the total
cable capacitance per foot. The formula below illustrates how to calculate the per-foot
capacitance.
