Design Ideas: September 2, 1996

Sometimes, using a component in an application other than its intended application can yield savings in design time, component count, cost, and complexity. The MM74C922N, originally designed to detect, debounce, and encode 16-key switch matrices, contains most of the elements needed for a full-function magnetic-tape-transport motion-control circuit (Figure 1). The IC replaces more than nine packages of CD4000 flip-flops and random logic and provides everything but a motion-sensing circuit. The MM74C922N provides the take-up, rewind, and play functions; simple external circuitry provides the motion-sensing function. A transport with motion sensing is gentle on the tape and adds the ability to change modes without going through stop mode. The cue/load function for easier tape editing and loading is almost free in this design; its cost is only an additional switch.

The MM74C922N combines all elements needed to scan detect, debounce, encode, store, and transmit contact closures in an X/Y switch matrix. The IC combines these functions in a way that makes it useful in stand-alone applications and as a bus-oriented system device. An internal RC oscillator (or an external frequency source) and scanning circuit poll the X/Y switch matrix for switch closures. The open-drain column-scan outputs allow the use of external open-drain, wired-OR logic, in addition to switches, as input devices. The encoder-row inputs have internal current-source pull-up devices. You can augment this current with an external current source if the application requires it.

An internal charge-pump integrator functions as a debounce circuit during switch make-and-break operations. The debouncer strobes the internal data latches, controls two-key rollover operation, and produces a data-available (DA) output strobe on switch closures. The DA line signals that the encoded 4-bit binary-output word corresponds to the column and row connected in the matrix. The three-state data output is LSTTL-compatible. Five momentary pushbutton switches command the tape transport. For these controls, the encoder's corresponding output codes (ABCD) are Stop 0000, Cue 0001, Playback 1001, Fast Forward 0101, and Rewind 0011. Each bit of the code corresponds to an active motor or solenoid.

The codes first latch internally in the MM74C922N. If the tape is stationary, Q₁ clocks the code for the next active operating state of the transport into MM74C175N's 4-bit latch. The latch outputs control relay drivers that supply power to the transport's motors and solenoids. Motion sensing occurs when a zebra-segment strobe disk mounted on the take-up motor's brake drum excites a reflective optical sensor in the brake housing. When the tape is in motion, pulses from the optical sensor, amplified by Q₂ and Q₃, retrigger a CD4047N monostable/astable multivibrator that is used as a frequency discriminator. If you select a different mode while the tape is moving, transistor Q₄ automatically clears the 4-bit latch to stop the tape.

Rotation of the strobe disk retriggers the monostable multivibrator, preventing the pending motion command at the keypad-encoder output from clocking into the latch until the tape speed either falls below a safe minimum or stops. Tape motion resumes when the multivibrator times out, clocking the new command from the keypad encoder into the 4-bit latch. Magnetic-tape transports benefit from added fail-safe measures. Transistor Q₇, in parallel with the stop switch, selects the stop mode and transistor Q₆ resets the 4-bit latch at power-up. Transistor Q₅ and associated resistors are a final fail-safe reset circuit. If conflicting motion-control codes occur at the same time (an event that has adverse consequences for the tape), Q₅ resets the 4-bit latch, thereby stopping the tape. Several circuits in the design perform double duty to cut component count. The relay-driver transistors also control LEDs for visual feedback of the operating modes. The LEDs also form part of the voltage-dropping network for the relays. (DI #1918)