Specifications

20
6.3 POTS/DSL Splitter
Rather then using the microfilters at each non DSL device I installed a
POTS/DSL splitter.
When the business line exits the secondary lightning protector it runs to a
Siecor (now Corning) POTS/DSL splitter. The splitter includes a low pass
filter that isolates the voice phone line from the high frequency DSL signals.
The splitter has two outputs; DSL is connected directly to the DSL modem
the phone output connects to inside phone wiring.
The splitter contains a half-ringer test circuit connected to thee phone side of
the splitter. The half-ringer in the splitter allows the one in the NID to be
removed.
6.4 Modem Access Adapter
If the DSL line fails the router automatically connects to the dialup ISP. We
wanted a way for the modem to have access to more then one line and to
prevent mutual interference between the modem and phones. This
maximizes the chance of completing the call while reducing cost by
eliminating the need for a dedicated modem phone line.
When the modem initiates a call the adapter detects the off hook condition
and searches for an idle line. If it finds an idle line it disconnects the phones
before connecting the modem. As long as the modem is in use the phones are
disconnected preventing them from interfering with the modem. If all lines
are busy the modem never receives dial tone and retries the connection
attempt later. This prevents the modem from trying to dial when all lines are
in use.
The adapter is connected to the primary personal line and the business line.
When the modem attempts to connect the adapter tests the primary personal
line first, if it is busy the business line is checked. The search order assumes
that during the day, when the business line is needed, the modem uses a
personal phone line. Since the two personal use lines are configured as a
hunt group when the first line is busy the call is automatically routed to the second. If the primary home
line is busy the data call is placed on the business line. This is most likely to occur after normal business
hours, when home phone usage is heaviest.
Two toggle switches control adapter features. The left hand switch enables or disables the device. It also
controls whether or not to search both lines. The right hand switch selects search order; either line can be
selected to search first. The red indicators show which phone lines are in use and which line the modem is
connected to.
The Modem Access Adapter was published as a Design Idea in the July 22, 1999 issue of EDN. A theory of
operation, schematic diagram, parts list and software listings were published.
Figure 21 Modem
Access Adapters
Figure 20 Splitter