BARD DISK CON'l'ROLLER HARDWARE OVERVIEif INTRODU CT ION The other major subsystem in the 486 system is the Hard Disk Controller board. This section provides a brief functional description of the circuit board and disk drive and also supplies formatting information. FUNCTIONAL DESCRIPTION Figure 5-4 is a block diagram of the hard disk controller. Data enters and leaves the board through connector J10 on the CPU board. Bits BD0 through 7 from the external bus are sent to a bus transceiver to create a three-way bus (D0-7) common to the sector buffers, the WD1010 and the external data bus. The sector buffers are a pair of lK X 4 RAM chips which hold 512 bytes of read or write data.
512 bytes is the size of one sector on the disk. Disk Drive Description The 20 Mbyte disk drive unit is a half-height random access 5-1/4 inch hard disk drive employing Winchester technology. Data is recorded on two 130rnrn diameter disks through four low force, low mass ferrite heads. Bead Selection Any of the four heads can be selected by placing the head's binary address on the two Head Select input lines. (Refer to Table 5-20 for Head Select 2 0 and Head Select 21 pin designations. ) 5-36