Termination voltage regulator

A project log for STEbus backplane

STEbus backplane

keithKeith 03/24/2021 at 18:030 Comments

The bus termination circuit from the specification is simple but the 2.8V regulator must also sink current when required. Common voltage regulators are only designed to source current. Examination of a 5-slot backplane reveals a working circuit.

Current flows through R3 to drive the NPN transistor TR1. The voltage reaches 2.8 volts, then the junction of the feedback resistor chain (R1, R2) reaches 0.7 volts and activates TR3 to drain drive current from the base of TR1 . TR3 also activates the PNP transistor TR2, to sink current from the 2.8 volt rail. Diode D1 helps stop both transistors conducting at the same time.

The whole circuit consumes about 20 mA, unloaded.

The termination regulator circuit should be useful for other buses.

Termination can be built on boards with DIN41612 connectors, but this adds the cost of connectors and occupies one or two slots.

Termination is recommended at both ends of a long bus (e.g. 10 to 21 slots), but a short bus (e.g. 5 slots) will work safely with termination at one end.