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Forums > C64 Coding > 1541 datarate
2012-05-15 01:16
Repose
Account closed

Registered: Oct 2010
Posts: 140
1541 datarate

Hi
I need help to understand the very low level 1541 disk format. From http://www.zimmers.net/anonftp/pub/cbm/schematics/drives/new/15.. I see that Y1, the system clock (which I'm guessing is 16MHz, cause a clock part is listed in the 1541-II service manual), goes to a /2 and /8 divider, configured in cascade, which means 1MHz is output. What I need to know is the actual bitrate of the outer zone. Inside c= dos says the divisor is set to /13 to /16, is this from 1MHz? Also the wikipedia article on 1541 lists bitrates of 16MHz/4/13=307KHz.
It's easy to answer, if you've written a fast loader, how long do bytes pass bye (GCR bytes)?
If it's right, 13*8/4=26 cycles per byte in track 1.

also do you know which bit is written first to the disk? lsb or msb?


2012-05-15 08:04
j0x

Registered: Mar 2004
Posts: 213
Yes, the 1541 uses 26, 28, 30 or 32 (1MHz) cycles per byte when writing, depending on speed zone. The VIA's shift register is msb-first (left-shifting), as far as I know.
2012-05-15 18:55
tlr

Registered: Sep 2003
Posts: 1535
See also here: Drive coding: safe maximum time between $1c01 reads? and here: Job codes'n buffers
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