Morrow - HDC-DMA Hard Disk Controller
This was another well designed S-100 Hard disk controller board.

This Hard Disk/Direct Memory Access (HDCDMA) Hard Disk Controller is a single board S-100 bus subsystem. It can communicate with up to four 5 1/4 inch or four 8" hard disks. The HDCDMA will support a mixture of types of drives, but all must have the same transfer rate. Variable designs, capacities and sector formats are possible for each type of drive, allowing users to upgrade their systems with new drives as they become available.

The HDCDMA is an intelligent controller. The arrangement of data on the disk is not limited by a special purpose LSI controller chip. This allows system designers the freedom to specify sector sizes and interleave to fit the requirements of special applications.

The controller has its own 8X300 bipolar microprocessor operating at 7.16 megahertz which is used to supervise data transfers between disk drives and memory without the intervention of the main CPU. This relieves the main CPU of time consuming processes which include head positioning, rotational delays and the usual byte-by-byte transfer of data from the controller to main memory. As a result, transfers are faster and more efficient. Moreover, the main CPU has more time for data processing, and thus, supports more users and/or tasks.

The main advantage of the HDCDMA controller over almost all the others is its glitch free direct memory access channel. This advanced "channel" concept allows the controller to communicate with S-100 memory by "stealing" bus cycles from the main CPU, or using the bus in "burst mode" for ultra-fast data transfer. This idea of an intelligent I/O channel was first implemented by IBM on their famous 370 mainframes. Now, this powerful concept has been implemented on the S-100 bus.

The channel has the full 24-bits of memory addressing as described in the proposed IEEE standard for the S-100 bus. Also, a great deal of care has been taken in the design of the interface circuitry so that it conforms in every detail to this new standard and still allows the controller to work well with existing systems designed before the standardization effort was started.

The board has priority logic which allows it to contend with up to 15 other "temporary masters" which may also want to "steal" bus cycles from the main CPU (the permanent master).

The features associated with the intelligent channel on the controller make it especially desirable in multi-tasking and multi-user applications. In fact, many were tailored to enhance the performance of Morrow Designs new, powerful DECISION I multiprocessing IEEE 696/S-100 machine. DMA style controllers are an integral part of advanced microcomputer systems which incorporate many of the concepts previously supported only on mainframe systems, such as the IBM 370 series.


The manual for this board can be obtained here.  See here for some fine adjustments often need with this board.