Current source for RX50 media?

Warner Losh imp at
Sat Jun 25 00:54:56 CDT 2016

On Fri, Jun 24, 2016 at 10:20 PM, Ethan Dicks <ethan.dicks at> wrote:
> My usual 5.25" drive (I have several) is the Teac FD55GFR - strapped
> for RX33 use with DEC controllers or strapped slightly differently for
> PC use (I know the directions are floating around)

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

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...

> I'll look deeper into those issues when we find some media.

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

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.


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