<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><meta http-equiv="Content-Type" content="text/html; charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">This sounds vaguely like FM or MFM encoding or one of its relatives.<div class="">The wikipedia article is informative: <a href="https://en.wikipedia.org/wiki/Modified_Frequency_Modulation" class="">https://en.wikipedia.org/wiki/Modified_Frequency_Modulation</a></div><div class=""><br class=""></div><div class="">MFM was instroduced in 1970 for the IBM 3330, which might be too late.</div><div class="">Or it could be FM, which has a transition in the middle of a bit cell for 1’s and a transition at the end of a bit cell for a 0 if it is followed by another 0.</div><div class=""><br class=""></div><div class="">FM has 1.5 average transtions per bit, which seems right.</div><div class=""><br class=""></div><div class="">The rew encoding mode might generate the data for the write head directly, at double the bit cell frequency.</div><div class=""><br class=""></div><div class="">-L</div><div class=""><br class=""><div class=""><br class=""><blockquote type="cite" class=""><div class="">On 2019, Jul 6, at 2:35 AM, Lars Brinkhoff <<a href="mailto:lars@nocrew.org" class="">lars@nocrew.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="">Hello,<br class=""><br class="">I'm looking into simulating a disk controller used with the MIT AI lab<br class="">PDP-10: a Systems Concepts DC-10. It looks like it's a custom made<br class="">channel interfacing to IBM disks. (It has been claimed those are 2311<br class="">or 2314, but the geometry doesn't match.)<br class=""><br class="">Formatting a disk pack uses a WRITE IMAGE command, which I believe<br class="">writes raw bits directly onto the disk surface. I'd like to decode<br class="">these to the data bits as seen by a normal READ or READ HEADER command.<br class="">I wonder if anyone recognizes the kind of encoding used?<br class=""><br class="">For every track, it goes like this:<br class=""><br class="">1. Wait for the physical start of the track.<br class="">2. Write 125460 bits of ones to cover the entire track.<br class=""> These will be overwritten by the following data.<br class="">3. Write a 108-bit header preamble:<br class=""> 111111111111111111111111111111111111111111111111111111<br class=""> 111111111111101010110101101011010110101101011010110101<br class=""> The preamble signals a header with with a zero, and then eight<br class=""> 10101 words.<br class="">4. Write a 108-bit header, converted to the on-disk encoding.<br class="">5. Write a 252-bit postamble:<br class=""> 111111111111111111111111111111111111111111111111111111<br class=""> 111111111111111111111111111111111111111111111111111111<br class=""> 111111111111111111111111111111111111111111111111111111<br class=""> 111111111111111111111111111111111111111111111111111111<br class=""> 111111111111111111111111111111111101<br class="">6. Write 55620 bits of alternating ones and zeroes. This is the data<br class=""> and checksum. These will come out as all 37080 zeroes with a normal<br class=""> READ command. Note the ratio between raw bits and data is exactly 1.5.<br class="">7. Write N bits of ones for the sector gap. N is computed from the<br class=""> length of the track minus the data fields, divided by the number of<br class=""> sectors.<br class="">8. Write next sector, go back to step 3.<br class=""><br class="">Now, in particular I'm curious about what encoding will transform a zero<br class="">bit to, on average, 1.5 "10" (or "01") bits?<br class=""><br class="">_______________________________________________<br class="">Simh mailing list<br class=""><a href="mailto:Simh@trailing-edge.com" class="">Simh@trailing-edge.com</a><br class=""><a href="http://mailman.trailing-edge.com/mailman/listinfo/simh" class="">http://mailman.trailing-edge.com/mailman/listinfo/simh</a></div></div></blockquote></div><br class=""></div></div></body></html>