Re: $1C0C (VIA2) in 1541

From: fachat (
Date: 2005-04-08 13:19:30

On Thu, Apr 07, 2005 at 08:09:29PM +0200, Patrycjusz R. Ɓogiewa wrote:
> Does anyone know what function has the PCR (mapped at $1C0C) of VIA2 
> inside 1541?

The PCR is the "Peripheral Control Register" of the VIA6521. It controls
the behaviour of the hardware pins CA1, CA2, CB1 and CB2.
Pleas see
To see what the pins do, look at the schematics of the VC1541, at
for example in the file 1541-33b-right.gif.

IIRC VIA2 is the drive controller VIA, so, the PCR controls the lines

CB1 - not connected
CB2 = MODE -> Output Enable (OE) of Gate Array
CA1 = BYTE -> Gate Array Pin BYTE, also connected to S.O. of the 6502
CA2 = SOE  -> Gate Array Pin SOE

IIRC MODE determines whether data is written to or read from the 
drive head, BYTE is an input that determines whether the read or 
write of a byte is finished. And SOE I cannot remember.
If you really want to know the exact meaning, you could analyze
the drive schematics of the early (long board) VC1541 (e.g. in 
file 1541-35.gif under the abovementioned URL) where there
was no Gate Array, but the logic was done in discrete logic (standard
TTL chips). I did this once, but this is is something like editing directly - you do it once and never again :-)


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