Apple II Computer Info
Path:
blue.weeg.uiowa.edu!news.uiowa.edu!uunet!comp.vuw.ac.nz!actrix.gen.nz!dempson
Subject: Re: Need info re accessing serial card
Organization: Actrix Information Exchange
Date: Wed, 3 Aug 1994 12:05:16 GMT
Lines: 103
>
> I always thought that the DIP switch on the SSC was similar to the power
> bar concept for your computer: It doesn't prevent you enabling interrupts
> through software when it is OFF (power bar OFF, computer ON), but it also
> doesn't force you to enable interrupts when it is ON (power bar ON,
> computer OFF). Someone please correct me if this is not the case.
Sorry, but that isn't correct. The interrupt enable switch (SW2-6)
must be ON for any interrupts to get through from the ACIA. It
disconnects the IRQ line if it is OFF.
Some of the ACIA's interrupt sources (DSR and DCD change) cannot be
disabled through software if you want to use the serial card (they are
disabled if the DTR output of the ACIA is turned off, but that disables
the whole chip). If the interrupt switch was on and you enabled the
DTR line (e.g. PR#2), any change on DSR or DCD would crash unless you
had an interrupt handler to catch it.
See my other posting on this thread for further details.
> Hmmm... I don't see the problem why everyone is so concerned about
> unlikely (or impossible) interrupts. Consider:
>
> The 6551 is only generating interrupts during the following situations when
> both receipt and transmission interrupts are enabled:
>
> o receipt register full
> o transmission register full
> o DCD state change
> o DSR state change
>
> The first two can be explicitly disabled on the 6551 through software.
>
> The third one will only occur if you are online and your host hangs up on
> you, or if you have your modem set to auto-answer and someone phones in
> with a modem carrier.
>
> The last one won't ever occur if you are not transmitting.
Either DCD or DSR may change state if you unplug the modem, or switch
it on/off.
> Remember the following two quirks about the 6551 as well:
>
> o You cannot send if you have the DTR signal set to false.
Apple II Computer Technical Information : Apple II Family Hardware Info
ftp://ground.ecn.uiowa.edu/2/apple2/miscinfo/hardware : May 2001 : 482 of 572
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