MAI Basic Four 2000 Series, Руководство по обслуживанию

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BFISD 8079
Thus an interrupt request level 1 is encoded on the 68010 interrupt request
lines (IPLO- through IPL2-). If the priority of the interrupt is greater than
the current processor priority, then the interrupt processing sequence begins.
The 68010 places the interrupt priority number on the lines carrying the three
least significant address bits. These are decoded by a 74LS151 l-of-8 data se-
lector (10K, CMB Logic Diagram, sheet 22).
For priority number 1, the CVPA- (CPU Valid Peripheral Address) output of the
data selector is low (active). In response to CVPA-, the 68010 internally gen-
erates a vector number that is determined by the interrupt level number. (The
vector number [when multiplied by four] is the address in the lowest 1K bytes
of main memory that contains the beginning address of the routine that will
handle the interrupt.)
The content of the vector number is fetched and loaded into the 68010 program
counter, and normal instruction execution commences in the interrupt service
routine. The program now can write a data byte to the data register in the
PI/T, for transmission to the printer.
3.2.18.2 Data Transmission to a Printer
The output data format from the parallel port is ASCII (American Standard Code
for Information Interchange); it's the same format used in the serial ports.
The parallel data output is asynchronous, and the data transfer rate is deter-
mined by the transfer program, which normally is interrupt driven. The drivers
and receivers in the parallel port circuitry are 74LS244s (4B and 5A). (Refer
to the CMB Logic Diagram, sheet 27.) These are connected to J13, the parallel
port connector, which has 37 pins. Table 3-8 is a list of the J13 signals. A
printer connected to the Model 4108 Base Unit parallel port typically will use
the so-called "Centronics parallel interface standard." Table 3-9 gives a de-
scription of the Centronics protocol.
3.2.19 Floppy Disk Controller
Two floppy disk(ette) drives may be mounted inside the Model 4108 Base Unit of
the MAI® 2000 SeriesDesktop Computer System. The driven diskettes are 5.25
inches in diameter, contain 96 tracks per inch (TPI) of diskette radius and are
double-sided. Controller circuitry to support both floppy drives is provided
on the Central Microprocessor Board. The circuitry provides for soft-sectored
and double-density recording techniques. Figure 3-12 shows a simplified block
diagram of the controller, and the following paragraphs describe the circuitry,
beginning with a very brisk review of a few basic disk recording principles.
3.2.19.1 Soft Sectoring
Soft-sectoring means there are no sector holes in the diskette to identify the
head position with respect to a track. Instead, a single hole provides a phys-
ical reference point, and sector information is recorded magnetically on the
disk within each sector.
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