Abstract: A programmable delay line with digital input to a two-part digital-to-analog converter structure to define an equivalent resistance at a pull-down node. Preferred embodiments are configured as two identical halves. The outputs of the two halves are combined to produce an exactly symmetrical waveform. This is particularly advantageous in a programmable delay line, since this architecture assures that control changes which change the delay will not also introduce asymmetry into the output waveform.

Abstract: An interface for current mode to Manchester encoded serial signals includes symmetric positive/negative doublet pulse generation. The doublet pulse generator provides a pulse duration held constant by feedback and a voltage/current reference and a minimal gap between the positive and negative polarity pulses by using the same signal to both turn off the positive pulse driver and turn on the negative pulse driver. Fault detection by sensing of asymmetry in received pulse uses a comparator with both high and low threshold input devices to provide minor asymmetry tolerance.

Abstract: An integrated circuit filter wherein two digital inverters are used, with cutoff frequencies which bracket the desired passband frequency. (The cutoff frequency of the two digital inverters is selected by changing their RC time constants.) The inverter with the lower cutoff frequency has its output connected to the reset input of a counter, and the inverter with the higher cutoff-frequency has its output connected to the clock input of a counter.

Abstract: A signal interface to a low-cost portable electronic token data module, which can be used with a wide variety of computers, including a tremendous variety of personal and other computers, as long as the computer includes an interface to RS232 (or some comparable stardard). The token has a one-wire-bus interface, implemented in a battery-backed open-collector architecture, which provides a read/wire interface. The communication protocol expected by the token has been specified so that the token never sources current to the data line, but only sinks current. The communication protocol also includes time-domain relations which are referenced to a very crude time base in the token, and the system must preserve timing relations which will be satisfied by tokens in which the time base takes on any of the wide range of forseeable speeds.