Calling all DOS-heads

From: Jim Brain <>
Date: Thu, 29 Mar 2012 19:54:24 -0500
Message-ID: <>
I bow to the knowledge of those who understand DOS better than I, but I 
am confused about the operation of the ATNACK circuit in the 1541.

Now that I finally have a good PC-based logic analyzer, I submit two 
traces for perusal:


If I then remove pin 10 of UB1 (the '06 that outputs the result of the 
ACK circuit onto the DATA line), I get this trace


All signals are tapped at the 6522 except DATA_ACK which is tapped off 
of UB1:pin 11 (high there means a low on the DATA line)

What I am seeing:

   * ATN_IN goes high (at the 6522)
   * it takes 444uS to bring DATA_OUT hi (which would pull DATA low)
   * command data sent
   * ATN_IN goes LOW
   * data is sent
   * At 7.4ms, ATN_IN goes high again
   * ATNACK is already low, so DATA_ACK goes high
   * DATA_OUT is already hi, so all is well
   * Than, at 9.443mS, ATN goes high again
   * DATA never goes high, ever
   * ATN_IN cycles a few times (zoomFLoppy), trying to get a response.

I don't understand why the DOS never notices ATN at 9.443ms

I am looking through the DOS routines, but it's slow going for me.  
Maybe someone with a bit more DOS knowledge can shed some light on it.  
I can create more traces if needed.


Jim Brain

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Received on 2012-03-30 01:00:05

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