Modem-access adapter reduces interference

Thomas Schmidt, Schmidt Consulting, Milford, NH -- 7/22/1999

The modem-access adapter (MAA) in Figure 1 has features that reduce problems when modems and phones share the same line. The design also overcomes the shortcomings of modem interference protectors. The design eliminates mutual interference between phones and modems, is compatible with the line-usage indicators on multiline phones, can search for an idle phone line in multiline installations, draws minimum power, and requires no changes to a PC modem's hardware or software.

Most residential PC users connect to the Internet using a dial-up modem. In most cases, the modem and voice calls must share one phone line. This sharing presents a problem because you can't use the modem and the phone at the same time; picking up an extension disconnects the modem. Many sites have more than one phone line. A computer has a better chance of connecting to a phone line if it can search multiple lines to find one that is idle.

Vendors of modem interference protectors claim the devices eliminate interference between extension phones and computer modems. The devices function by monitoring the voltage between the phone line's tip and ring leads. When all phones are on their hooks, this voltage is typically >48V. Picking up a phone closes the tip-and-ring circuit, which drops the voltage to approximately 7V. The protector monitors the line voltage. When the voltage drops below a certain threshold, the protector disconnects the phones that are connected to it. This action prevents an active phone from interference when you pick up an extension.

Unfortunately, these protectors have two serious shortcomings. First, the device needs to be in series with the phones and modems it is protecting, which requires one device per phone. When you use the device in this manner, you cannot use more than one extension at a time. This situation prevents you from placing a call on hold and picking it up on another extension. Second, you can split the wiring into two circuits: one for phones and one for the modem. This split wiring minimizes the number of protectors you need and allows multiple extensions to pick up the same call. Unfortunately, regardless of how you wire the protectors, the voltage drop across the protector confuses the line-busy indicators on most multiline phones. Multiline phones no longer indicate when a line is in use.

The MAA in Figure 1 does not suffer from these problems. You must connect the MAA in series between the telephone-company central office and the phones and modem the adapter is protecting. The most convenient location for the MAA is in proximity to the phone-company network's interface demarcation point. Simply disconnect the inside phone wiring from the demarcation point and plug this wiring into the MAA. Then run a dedicated phone line between the MAA and computer modem.

The MAA consists of control (Figure 1a) and data-path (Figure 1b) subsystems. Phone lines source high voltages and are balanced with respect to ground. The design uses electromechanical relays to switch the phone lines while providing an isolated control path.

The heart of the control circuit is IC₁, a PIC16F84 (Microchip Technology, www.microchip.com) low-cost RISC µC with flash memory. Flash memory makes this part ideal for development. The MAA program is approximately 100 words long.

When the modem is idle, relay K₃ supplies power to the modem local loop (Figure 1b). When the modem goes "off hook" to place a call, relay K₆ senses current flow and wakes the CPU. The CPU reads the state of the hunt-order switch, S₂, which is a physical switch that allows the user to select the phone-line search order.

Relay K₄ monitors Line 1 (L1), and K₅ monitors Line 2 (L2). When a phone is off hook, loop current causes the appropriate line-sense relay contact to close. The CPU uses this information to determine if the line is in use. Assuming the hunt-order switch is in the search-L1-before-L2 position, the CPU reads the state of L1. If L1 is idle, the CPU energizes K₁ using the L1_CNTL signal. Energizing K₁ disconnects the extension phones from L1 and routes L1 to the normally open contacts on K₃. Then the CPU energizes K₃, which connects the modem to L1.

If L1 is busy, the CPU reads the modem-mode switch. If the setting of S₁ enables the hunt mode, the CPU checks the other line for availability. If L2 is idle, the CPU energizes K₂ using the L2_CNTL signal. Energizing K₂ disconnects the extension phones from L2 and routes L2 to the contacts on K₃. The CPU then energizes K₃ to connect the modem to L2. If both lines are busy, the modem never gets a dial tone and eventually disconnects.

The telephone central office typically disconnects the phone line for a short time at the beginning of the dial tone. After the circuit establishes the connection, the CPU enters a long delay loop. During this time, the CPU ignores changes in the state of the line. At the end of the delay, the CPU checks to see if the line is in the expected condition. If it is, the CPU goes back to sleep. If it is not, the CPU processes the new state.

When the modem hangs up, relay K₆ deenergizes, which also wakes the CPU. When the CPU detects hang-up, it tears down the modem connection and returns the lines to their default state for voice.

The bleeder resistors R₁ through R₄ discharge the phone capacitance and impress the line voltage on all of the extension phones to ensure that the telephone-busy indicators function normally. Relays K₄ through K₆ (Teltone, www.teltone.com) are specially designed to monitor telephone loop current.

The MAA device connects directly to the phone line and therefore must meet FCC CFR 47 Part 68 for protection of the public switched telephone network. Residential devices must meet FCC CFR 47 Part 15 Class B for limitation on unintended radiation. (DI #2389)

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