On May 27, 22:25, jkunz_at_unixag-kl.fh-kl.de wrote:
> It is no original DEC RXV21. It is a third paty product, "MXV21" made by
> "MDA". There is all the stuff on it to make it a complete controller.
> Read/write circuit, 2 x AM2901 with all the AM29xx stuff around it,
> TMS4044 SRAM, ... It has the usual 50 pin connector for 8" floppy
> drives. If I plug it into one of my QBus VAXen it is shown as RXV21 by
> the "show qbus" command.
Then it might actually have a formatter built in to its firmware, like the
Plessey and GR ones I mentioned in my earlier reply. Several companies
seem to have copied the GR system, and your description matches (though
that's probably not surprising).
On an RXV21, there are three bits in the CSR which determine the operation
to be performed when the CSR is written to. The commands in binary are:
000 fill buffer (ie transfer data from host to controler)
001 empty buffer (ie transfer data from controller to host)
010 write sector (using the contents of the controller buffer)
011 read sector (from floppy into controler buffer)
100 set media density -- see below
101 read status (updates the controller registers with drive etc status)
110 write deleted data (like 010 except with a special address mark)
111 read error code (used to access extended status registers)
On a DEC RXV21, the 100 code to set media density is only used to alter SD
to DD or vice-versa (according to whether bit 8, the DENSITY bit, is set in
the CSR or not). To do this, you set/clear the appropriate bits in the CSR
according to which drive you want, which density, etc, and at the same time
set bits 1-3 (the command bits) to 100. Then you poll the CSR until the TR
(Transfer Request, bit 8) is set. When it is, you write the value
(octal)111. The drive then rewrites the headers to indicate the
appropriate density, and writes zeros in the data fields. The disk has to
already have a valid format on it.
However, if you use a GR or Plessey RXV21, and reply with the value
(octal)222 instead of (octal)111, it formats the disk, which needn't have
previously had any recognisable format on it at all.
If yours is a copy of the GR RXV21 then it should not only be able to
format disks, but will work with double-sided disks or up to 4 single-sided
disks, not just two single-sided.
--
Pete Peter Turnbull
Network Manager
University of York
Received on Sun May 27 2001 - 16:15:55 BST