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Tarbell Electronics -- 1011A Floppy Disk Controller

Tarbell Electronics followed their first single density  FDC with this double density one.  

Tarbell 2022 FDC

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 board offered.

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 here. 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 S-100 Boards
Cassette Interface     Single Density FDC      Double Density FDC       32K RAM      Z80 CPU Board


This page was last modified on 06/06/2015