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AST RESEARCH TECHNICAL BULLETIN TB-0010 4-13-84
IRQ-7 PARALLEL PORT CONTENTION
IRQ-7 USED BY AST MULTIFUNCTION BOARDS
FOR PARALLEL I/O PORT OPERATIONS
In some of the earlier multifunction boards, where the parallel
port is an option and that option is not included on the board, a
problem is created.
When this option, parallel port, was not ordered, a IC (74LS174)
that governs the status lines for the parallel port was deleted
on the bill of materials. Unfortunately, this IC also controls
the IRQ-7 line. With this IC (74LS174) not present, it creates a
low condition, via 74LS125, which causes contention with other
devices trying to use IRQ-7. Example of circuit is attached.
The IC (74LS174) must be installed for proper operation. Refer
to schematics, at rear of user's manual, for correct location of
74LS174 IC.
AST Research suggests when working with dual in line packages,
that you carefully place IC's in socket so that the legs insert
properly into socket. Examine carefully after insertion and
verify that all legs of IC are seated properly in their
respective socket.
NOTE: As a portion of the data is in pictorial form, it is not
included here. To obtain a complete copy of this technical
bulletin, contact:
AST RESEARCH, INC.
2121 Alton Avenue
Irvine, California 92714
ATTENTION: Product Support
(Technical Bulletin)