Poke 11895 to change stepping rate in realtime on 8" drives

[lowrybt1]

The Dragon Byte Technical Manual for OSI disk interfaces indicates that disk stepping rates under 65D (at least for 8" drives) can be changed on the fly by poking a value into 11895. Poke 11895,3 sets stepping rate to 3ms. Poke 11895, 9 sets stepping rate back to 9ms. According to the manual, this might be necessary when using 'later edition' D-series and above Siemens 8" floppy drives. I guess this might be useful if a system with two 8"drives mixes older or newer Siemens or Shugart drives. Has anyone tested this before?

[Mark]

I'm not sure which version of OS65D was referenced for that poke. At least with OS65D V3.2+ that address seems to be the high order byte in the command lookup table for the SE command. See OS65D_Dissasembly_Manual

The head step seems to happen around $2683 with a delay value stored at location $EF, but you can't POKE that when running BASIC. Zero page is constantly swapped in and out by BASIC, so that location changes meaning. Step delay at $EF seems to be set once at $22B9 during OS load, and swapped in & out with BASIC's zero page storage at $2F79+ with $EF being stored at $3068. BASIC swaps it back when making OS calls.

So a poke at 12392 might be the address you are looking for on OS65D V3.2 & 3.3 with BASIC running. I don't have older OS versions for 8" systems to see what they did.

I wonder why a slower step rate would matter to newer drives and make them operate "ineffectively"? Maybe I misunderstood their statement. Certainly setting it faster for drives that can handle it would make seeks faster.

(My OS65D version seems to use a value of 8.)

[lowrybt1]

Thanks Mark. This is not the first time something in that technical references seems off. Another example: the reference illustrates certain trace cuts and jumpers for Siemens FDD-100-8D/E drives that will be used as B drives with OSI machines. Looking at 3 FDD-100-8D drives used as drive B on C8 and C3 systems, not a single one used the same trace cuts and jumpers shown in the Dragon Byte book.