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Ie: GPIB Interface

Dedicated chips exist, but older equipment use discrete components for this, where the output is often an open-collector gate/inverter. The purpose ofthe pullup is something I understand, but what is the purpose of the 6k2 pulldown? Is it to terminate the bus with a certain impedance? Just curious.

The GPIB control-lines are a type of wire-or, so that any or all devices can hold down (assert) say NDAC to control the bus speed. Thus the slowest device controls the speed of transfer.

Image is taken from here: http://bitsavers.org/pdf/hp/hpib/TutorialDescrOfHPIB.pdf

MarkU
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AndersG
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1 Answers1

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Don't think of RL2 as a pulldown. Think of RL1 and RL2 together as a Thevnin termination.

They present an impedance of about 2k to an effective voltage of about 3.3 V.

As GPIB was designed to use TTL levels, a high voltage of 3.3 V is all that's needed.

Neil_UK
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