114 IN THE BEGINNING

modules that would be sold only as part of a system. For example, all of the DEC core memory circuits have been in the G-Series because a core memory system is sufficiently complex that a cookbook approach using a standard series of modules is not appropriate. The G-Series is still actively used today for circuits other than logic, generally in peripheral devices such as disks, tapes, and terminals.

Like the A-Series and G-Series, the W-Series (white handle) is still manufactured and is used to provide input/output capability between Flip Chip modules and other devices. Lamp drivers, relay drivers, solenoid drivers, level converters, and switch filters are included in this family, but the only modules used widely today are those modules which include cable termination modules and blank boards upon which the user can mount integrated circuits and wire-wrap them together.

While the W-Series modules provided a variety of interface capabilities, their circuitry was still too fast for typical industrial applications. Computer logic, by its very nature, is high speed and provides noise immunity far below that required in small-scale industrial control systems located physically close to the process they control.

Unfortunately, industrial electrical noise is not predictable to the nearest order of magnitude. Thus, attempts to solve noise problems with high level logic, whose voltage thresholds were merely a few times greater than computer logic thresholds, did not work well.

A new series of modules was developed, the K-Series (with blac(K) handles), which relied on a combination of voltage, current, and time thresholds to protect storage elements such as flip-flops and timers from false triggering. Since industrial controls typically interact with physically massive equipment which moves slowly relative to electronic speeds, time thresholds are particularly attractive. There are four ways of exploiting these:

1. Using basic 100 KHz slow-down circuits everywhere.

2. Making optional 5 KHz slow-down circuits available.

3. Providing transition-sensitive (edge-detecting) circuits with hysteresis to allow additional discrete capacitor loading of the input when all else fails.

4. Replacing the conventional monostable multivibrator or "one-shot" circuit with' a timing circuit which has both a low impedance and hysteresis at the input.

The hardware for the K-Series was specifically designed to fit the NEMA (National Electrical Manufacturers Association) enclosures traditionally used with relay implemented industrial controls. The K-Series used the same connectors as the other Flip Chip modules, however. Sensing and output terminals were provided with screw terminals and indicator lights, and appropriate arrangements were made to interface with 120-volt ac devices. Wire-wrap terminals were protected from external voltages but were available for oscilloscope probes. Magnetically latched reed relays and diode arrays that could be programmed by snipping out diodes were provided as memory elements that would retain data during power failures.

Gating in early K-Series modules was accomplished with discrete diode-transistor circuits such as that shown in Figure 19. Other K-Series modules used integrated circuits for the logic functions. In these designs the inputs to the integrated circuits were protected with filter/trigger circuits which filtered out the noise and then restored the fast risetimes required by the integrated circuits. Outputs were protected from output-induced noise and converted to standard K-Series signals by circuits similar to those used in the discrete logic gates.