[ct-user] Keying from High IRQ COM Port?

[Waldemar Krzok]

> Usually, LPT1 will use IRQ 7 if it is set up in a bidirectional mode 
> (SPP, ECP, EPP).  This is necessary for some printers and for scanners and other
> specialized hardware.  Otherwise it is not needed.  Soundcards 
> or LPT2 if installed tend to end up on IRQ5.  My favorites for
> COM3 and 4 if you have enough IRQ's are IRQ 10 and 11.  IRQ 9 
> can sometimes be used, but like Bob said, it is used to map the
> higher IRQ's to IRQ2 and sometimes it doesnt
> work well as you are actually sharing the IRQ at that point.

Just to clear: you are not sharing inerrupts at this point. IRQ2 is 
used for chaining the 2nd interrupt controller, IRQ9 is physsically 
mapped to IRQ2, i.e. the line 2 from 2nd controller is connected to 
slot connection, where in ancient time (IBM PC & XT) line 2 from 
the only interrupt controller was connected.
Despite the fact, you are not sharing interrupts, programs can run 
havoc. It "thinks" it uses IRQ2 and resets only 1st controller, and 
the 2nd sends requests ad finitum.

Waldek, DL7ANQ & SO6ANQ

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