Posted By
BSZ
on 2023-08-10
16:22:38
|  Re: 1551 diagnostic / 1551test.prg / repair possible?
I'm not sure what you've exactly tried so far, but a few things come to mind.
If the 1551 drive has unit number 8, the base address of the paddle is $FEFx. Specifically, $FEE0..$FEFF, this is the range over which the 8 registers (6 used, 2 unused) of the TIA are repeated 4 times. But the machine's kernal uses the $FEF0..$FEF5 addresses. The address range of a drive with unit number 9 is $FEC0..$FEDF, of which the addresses used are $FEC0..$FEC5. (Is the test testing a non-existent drive's paddle?)
But... The error "21 READ ERROR 18 0" is provided by the drive, so if you see this, the communication with the drive is OK. Probably the paddle is fine. This error code means that it has failed to read the 18:0 sector because it cannot find a synchronizing sequence. (The 18:0 is the BAM of the disk, the drive always start with this for a new disk.)
This error occurs if the disk is missing or badly formatted. If the disk works fine in another drive, there is a problem with this 1551 drive. Unfortunately, this is often caused by mechanical failure; the 1551 has the "infamous" D500 mechanism, which has a head that tends to fail. (This is easy to check with a multimeter, but if you haven't done it before, it may not be a good idea to learn on this.  )
If you don't have a good disk / drive handy, try a disk format:
HEADER "NAME",IXX,D0
If the format does success, the drive may be fine. If it fails, the error message may be interesting.
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