Electronic Design

RS-232 Port Powers Frequency Counter

As a source of power, the EIA-232 port has found its way into many applications unrelated to data communications—most notable being the serial mouse. An unusual innovation among RS-232-powered systems is the frequency counter, as seen in the figure. This circuit hard-limits the signal and, by driving a handshake line such as CSR, uses the resulting high/low information to trigger the interrupt of a PC’s COM port. The COM port data lines aren’t used because the UART framing would interfere with edge detection.

If the computer can measure the time between interrupts it can measure the signal's frequency. This technique is useful in simple telemetry, in which the frequency represents a voltage, and in more complicated situations such as the demodulation of a baseband signal from a radio receiver. Many shareware programs available on the Internet use this technique.

The output levels delivered by EIA-232 transmitters have scaled down over the years. Equipment has evolved from bipolar line drivers powered by external supplies to CMOS line drivers powered by on-board dcdc converters (pioneered by the MAX232). Many of these later-generation CMOS chips meet the ±5-V specification for transmitter voltage by a margin of no more than 1 V.

The newer ICs work fine in the intended EIA-232 applications, but their minimal voltage levels pose a challenge to designers intent on stealing power for other applications. As a further complication, the frequency counter operates on 4 V, but also must withstand ±15 V (in case it gets plugged into an older PC).

The circuit shown meets all of these requirements. IC1 is a lowdropout regulator that generates a 4-V supply rail from the EIA232’s DTR signal. IC1 can withstand the full 15-V transmitter voltage, and its “reverse-battery” protection can alsohandle the maximum negative voltage (−15 V), which eliminates the need for a blocking diode and its associated voltage drop.

IC1 powers the dual comparator IC2. Comparator “A” is used to hardlimits the input signal, and comparator “B” detects whether the signal is approaching the supply rail. Comparator “A” compares the ac-coupled audio to a pseudo-ground created by R3 and R4, and drives the EIA-232 port’s CTS pin.

The comparator’s output drive is sufficient to pull the EIA-232 receiver’s worst-case low input impedance (3 kΩ) above the receiver’s required minimum (3 V) input voltage even when powered by 3.9 V. The near-ground output of comparator “A” violates the ±3-V minimum for signal levels at EIA-232 receivers, but the circuit works because the trip thresholds in these receivers are roughly TTL-compatible.

Resistors R5, R6, and R7 provide ±0.2-V of hysteresis (10% of the signal swing). If the signal reaches 3.6 V, the “B” comparator trips, causing current flow in the LED that indicates when there’s excessive input voltage. Input voltages beyond the supply rail cause current flow in the protection diodes, but not false outputs from the comparators.

Most of this circuit’s supply current drives either the LED or the EIA-232 input. The circuit never does both tasks at once, so the peak supply current is about 3 mA. The CMOS ICs themselves consume very little power. Typical supply currents are 30 pA for IC1 and 14 pA per comparator in IC2.

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