Interesting issue with 1541 ATN line

From: Jim Brain <>
Date: Thu, 10 Oct 2013 21:07:11 -0500
Message-ID: <>
Playing with ATN inhibit at the 6522 IC.

If I disconnect ATN input to 6522 and ATNACK from 6522, and hook the two 
pins from the socket together, (thereby tying both pins of the 74'86 to 
the same value), I can create a device not present condition.

The interesting behavior occurs if I have previously used the disk drive.

If I perform this mod immediately after reset, the circuit works as 
If I perform it after a disk access, the computer locks up waiting for a 

I have verified it's not the computer, but the drive.

I put a LA on the signals.

After reset, with my mod,, a bus request causes the ATN and ATNACK lines 
to cycle for a while and then go low.
After a disk access, then my mod, a bus request causes ATN and ACK to 
cycle for a bit and then go high.

I'm struggling to understand the differences.  I assume the code is 
doing something slightly different at IDLE after a disk access from what 
happens after RESET.


Jim Brain

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Received on 2013-10-11 03:00:04

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