Sharp MZ-5500 Tehnical Manual page 54

The MFD drive unit has three major actions of write, read, and seek whjch
are directly controlled by the ßPD765 Floppy Disk Controller (FDC) shown in
Fig.7-1. As the side of disk (front or reverse side), track, sector, and
command (write, read, seek) to the FDC are specified from the CPU by the
software, the FDC automatically contro~ the MFD drive unit.
The 8237 DMAC is used to transfer read/wtite data between the FDC and the
memory to perform fast DMA transfer. The following are required in order
that the FDC reads from or writes to th e disko
(1)
The DMA request signal (DRQ) is issued from the FDC to the DMAC anJ
VFO/data separator (*1).
(2) As the DMAC receives DRQ, HRQ (request) is sent from the DMAC to the
CPU.
(3) As the CPU receives HRQ, the bus line is opened, and HLDA is returned.
To which the DMAC returns DACK to the FDC.
(L,)
Data are transferred between the memory and FDD via FDC under th e blls
control by the DMAC.
The ~receive circuit from the MFD drive to the FUe is simplified by the use
of the SED 9420 VFO/Data Separator
le.
As it internally incorporates the
window clock (*2) by which the read data signal is fetched from the floppy
disk drive which is then divided into the clock pulse and data pulse and the
timer circtiit. This controls the MFD motor stop signal.
MFD drive select signal 1s sent out through the 1/0
~ort
which is
incorporated in the AY-3-8912 Programmable Sound Generator (PSG).
*1): VFO *2): Window
Read data output from the MFD are composed of the data pulse and the clock
pulse, and they must be separated from each other by the reliable data
window, in order to obtain the data of a low error rate. lt is the VFO
circuit that is designed for that purpose. Read " data input from the MFD
delivered by the VFO is rearrollged to tlle mid-point of the window, hefore
being sent out as a da ta signal.
O l i l O G
f)
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V
f)
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DA, T A
~ ~ ~
Window
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