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ANR Info-Pending Latch

IP.com Disclosure Number: IPCOM000047561D
Original Publication Date: 1983-Dec-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 1 page(s) / 11K

Publishing Venue

IBM

Related People

Gorga, KJ: AUTHOR [+2]

Abstract

The IBM 3277 display terminal interface (ANR) is a poll-driven type wherein the control unit may poll a terminal at a variable rate. When a terminal has status to report, it sets bit 6 (info-pending) in the Status Word. The Control Unit then looks at the Device Check or AID character bits. These bits remain valid on subsequent polls until an Acknowledge bit is returned by the Control Unit. It is possible, under conditions of light computer system loading, that the Control Unit could return the acknowledge bit and poll again before the microprocessor could reset the info-pending bit. As a result, the Control Unit would see info-pending again and interpret this as another read request.

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ANR Info-Pending Latch

The IBM 3277 display terminal interface (ANR) is a poll-driven type wherein the control unit may poll a terminal at a variable rate.

When a terminal has status to report, it sets bit 6 (info-pending) in the Status Word. The Control Unit then looks at the Device Check or AID character bits. These bits remain valid on subsequent polls until an Acknowledge bit is returned by the Control Unit. It is possible, under conditions of light computer system loading, that the Control Unit could return the acknowledge bit and poll again before the microprocessor could reset the info-pending bit. As a result, the Control Unit would see info-pending again and interpret this as another read request. In order to prevent this from happening, a provided hardware latch is set by microprocessor control to report info-pending and is reset in hardware by the acknowledge bit. This solves any possible timing conflict that would arise out of the use of a slower speed microprocessor.

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