On Fri, 24 Jun 2016, Ethan Dicks wrote: That would be 80 TRACKS at 96 tracks per inch. Those would be the 300 Oersted disks, same diskettes as "360K". Do NOT use the 600 Oersted "1.2M"/"HD" disks. Being Single Sided, it really won't hurt to use just the first side of a Double Sided disk. 80 track V 40 track both call for the same chemistry of the coating, but you should probably bulk erase whatever you get. If you use a PC to format them, the requirements are essentially the same as for a "720K" 5.25". Formatting can not be done with a "360K" (40 track) drive, but can be done with a 1.2M drive, or with a 720K 5.25" (which aren't common in the PC world). There are a couple of potential complications to using a 1.2M drive, that can be gotten around in software. (96tpi track spacing, same write current, rotational speed and data transfer rate as for 360K) We all have our favorite and hated brands. Ones you buy now, would likely be used or New stale stock, but that probably won't be a problem if they were stored well. -- Grumpy Ol' Fred cisin at xenosoft.com

On Fri, Jun 24, 2016 at 10:20 PM, Ethan Dicks <ethan.dicks at gmail.com> wrote: The RX-50 is a 80-track version of the 360k IBM-PC 40 track floppies. It was formatted at 10 sectors per track with smaller inter-sector padding bytes than normal. It uses the same rotation rate and data transfer rate as the older drive, which is different than the AT formatted floppies use. So RX-50's use 250k transfer rate, 80 tracks, usually 1 side (though the controller supports 2 sides, the physical RX-50's don't). 512 byte sectors. 16 bytes between sectors (as opposed to either 35 or 42 for IBM-PC floppies) with 46 bytes format gap (as opposed to 80 for IBM-PC floppies). This is how you got 400k out of a single sided 80 tracked disk. IIRC, the rotation rate is 300 for RX-50 and old IBM-PC vs 350 for the IBM-AT drive. All these niggling differences cause problems... One thing to avoid is disks with hub-rings. On the Rainbow they caused nothing but grief. You might not be able to at this late date, but if you have a choice. Something like http://www.ebay.com/itm/NEW-BOX-OF-10-MAXELL-MD1-DD-SINGLE-SIDED-DOUBLE-DEN… may work. These are single sided double density but list 96tpi, which means they will go to 80 tracks easily and should be able to be formatted in the Rainbow for use on the PDP-11. The 11's controller, like I said, won't format. Hope this helps. Warner

On Sat, 25 Jun 2016, Warner Losh wrote: Your info is good. The RX-50 and the IBM "360K" when in their correct drives, spin at 300 RPM, with a data transfer rate of 250K bits per second. The "360K" is 40track/48tpi; RX-50 is 80track/96tpi The IBM "1.2M" diskettes (and 8"DD) spin at 360 RPM, with a data transfer rate of 500K bits per second. 80track/96tpi It gets more squirrelly when you want to use a "360K" disk (or RX-50) in a "1.2M" drive! You can either change the drive speed to 300RPM (on drives that support both speeds) and use a 250K data transfer rate, OR (on drives that don't change speed) leave the speed at 360 RPM, and change the data transfer rate to 300K bits per second to compensate for the different rotational speed. (It can get confusing because "300" is both the rotational speed of the "360K" and RX-50, AND is the data transfer rate when compensating for 360RPM, AND is the Oersted (coercivity) of the disks for "360K" and RX-50. "360" is both the rotational speed of "1.2M" and 8", AND is the polpular nake for IBM's DSDD 5150/5160 disks) Since the RX-50 is DD with 80 track/96tpi (what the marketing people called "QUAD density"), a "360K" drive won't work. A "1.2M" drive is 80 track/96tpi, but is more commonly used at 360 RPM and density that the marketing people called "high" (which is what the marketing people were; you won't believe what the Superbrain/Intertec people called DSDD80track!) The RIGHT drive for RX-50 would be a 5.25" "720K" drive, which weren't commonly used on PC. Such as Tandon TM100-4, Shugart/Matsushita 465, or Teac 55F. ("1.2M drives are #475 or 55G. 55FG is a "1.2M" drive that is explicitly intended to also be able to do "720K". Some "1.2M" drives can switch between 300RPM and 360RPM, some are stuck at 360RPM and need a data transfer rate change to compensate) Because the "1.2M" drive needed to also be able to read "360K" diskettes, provision was made. 40 track V 80 track can be read just fine by using every other track, BUT because the track width is different a "1.2M" drive does not adequately cover/erase a wide ("360K" track that it tries to write over. (1/3mm track width at 1/2mm spacing V 1/6mm track width at 1/4mm spacing) Need an analogy? Two bicyles can ride on the tracks made by a car, but the bicycle tire-prints won't obliterate the car tire-prints. In addition, for "360K" in a "1.2M" drive, either the rotational speed or the data transfer rate needs to change. SO, . . . to to do RX-50 in a "1.2M" drive, we need the 80track/96tpi track spacing, but the "360K" RPM/data transfer rate. Proper: 300RPM with 250K data transfer rate, 96tpi. "1.2M" kludge: 360RPM with 300K data transfer rate, 96tpi (Note: the Tandon TM100-4 is an excellent drive for this, but the TM100-4M is 100tpi instead of 96!) RX-50 and "360K" diskettes both have a coercivity of 300 Oersted. BUT, "360K" is tested at 48tpi. If there are flaws between the tracks, they won't be spotted in testing, but will show up for RX-50! So, ideally, you want disks tested for 96tpi (marketing "quad density"), or GOOD disks without flaws, even between the tracks. In theory, any good "360K" diskette should do, but some folk have encountered errors on disks not tested for 96tpi. "1.2M" diskettes have a coercivity of 600 Oersted. When written by a drive expecting 300 Oersted, data longevity may be shortened. (Roytype "HD" with TRS80 had an average data retention of less than an hour) Do NOT use "HD" ("1.2M") diskettes for any RX-50 data that you would like to keep. -- Grumpy Ol' Fred cisin at xenosoft.com

On 06/24/2016 09:13 PM, Fred Cisin wrote: IBM-style PCs with 1.2M drives use a 300Kbit/sec transfer rate for double-density floppies. A tool such as Dave Dunfield's IMD will work just fine. The last time I duped a bunch of RX50s, I used Verbatim "DSDD" media. Forget about searching for "96 tpi" or "100 tpi" branded floppies--all that indicates is that verification was performed as part of manufacturing QA at the indicated track density--it doesn't really have a bearing on the results. If a DSDD floppy is good at 360KB, the chances are overwhelming that it'll be fine in DSQD mode. As far as ring-less hub floppies go, those are difficult to find and probably quite old, unless they're specifically labeled (complete with little arrows) for RX50 use. Hub rings differ between brands in thickness; the "clear" type are generally thinner than the opaque (black or white) ones. As an aside, the reinforcing ring was originally introduced for early FH 5.25" drives that didn't have circuitry to sense when a disk was being inserted and activate the spindle motor. The result that a floppy could be clamped off-center and mangle the hub area and the data on the floppy itself. I remember that Dysan and Verbatim started doing this when the old un-modified Micropolis drives were in use. Initially, it started with a supplied jig and a bunch of self-stick hub rings. In a short time, the jig and rings were dropped and floppies simply came with the rings applied. As most 8" drives of the time have AC line motors that spin continuously, this mis-registration was never a problem there. When half-height DC-spindle motor 8" drives came out, the problem was already known in the industry, so 8" floppies don't have the rings. 1.2M (DSHD) floppy drives came much later, after the problem had been diagnosed and remedied, so those don't *unually* have hub rings. FWIW, Chuck