Powerup Reset Circuit

 

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Powerup Reset Circuit
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Here is a description of the powerup/reset circuit that is used on some of the small PDP-11/23's. This circuit can probably be used to adapt a PC power supply to a Qbus card cage.

The powerup reset circuit has to generate BPOK and BDCOK. These are supplied to a 10 pin header on the back of the backplane.

BDCOK --o  o-- BHALT
CS3L  --o  o-- not connected
CL3L  --o  o-- GND
(key) --   o-- SRUN
BEVNT --o  o-- BPOK

CS3L and CL3L are not used. See the Qbus spec for a detailed description of the other signals. Unfortunately, I could not find the Qbus spec on the web. The overview description of Qbus can be found here.

A 74LS14 and resistor/capacitor networks are used to generate the signals. There are two circuits, basically identical except for the values of the timing capacitors. 

 I added a switch that shorts out the 10 uF capacitor. This does basically the same thing as the reset switch on a PDP-11 front panel.

The reset pushbutton is in the lower left of the picture. Note the fans to the left of the card cage.

 

 

Last modified: December 31, 2009