Tarbell Electronics -- 1011A Floppy Disk Controller
Tarbell Electronics followed their first single density FDC
with this double density one.
The Tarbell Double Density Interface was an IBM soft sector floppy disk
interface using the Western Digital 1791/1793 Floppy Disk controller chip and
built to the IEEE S-100 Standard. This interface is very similar to the
1011A Tarbell Single Density board,
but with some added new features. These features included operation as either
single or double density, or both, direct memory access (DMA), and extended
memory addressing capability, with processor speeds of either 2 or 4 Mhz. The
on-board BOOTSTRAP feature allowed full system memory of 64K by using the
PHANTHOM line on the bus.
Software available to run the double density controller was the then widely used
CP/M disk operating system, and the then new MP/M multi-tasking operating
system. Both operating systems used an enhanced I/O system utilizing the new
direct memory access capability, and automatic density select features that this
The capacity of the Disk Operating System running under CP/M in Double Density
was 476 Kbytes for an empty disk, or 243 Kbytes Single Density for an empty
disk. The break down was as follows:
Double Density: The disk will appeared to CP/M as 77 tracks of 51 sectors, each
sector containing 128 bytes. Because the first two tracks were used by CP/M for
the operating system, there was be 75 tracks available for Directory and data
storage. This total space then equaled approx. 476 Kbytes.
Single Density: While running Single Density, the disk appeared to CP/M as 77
tracks of 26 sectors, each containing 128 bytes. The first two tracks were used
for the CP/M operating system, leaving 75 tracks available for Directory and
data storage, about 243 Kbytes of disk space.
The manual for this board can be obtained
Interestingly no schematic was provided with the manual. Fortunately Jerry
Mulchin, who worked for Don Tarbell at the time and designed both their Z80
and DD FDC board fowarded me
this one. (Jerry later went on to setup his own company
InterContinental Micro Systems).
Other Tarbell Electronics
Interface Single Density FDC
Double Density FDC
Z80 CPU Board
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