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2.9
THEORY OF OPERATION
The ACB-5500 provides a powerful mechanism for connecting up to
four ST506
compatible disk drives to a host computer via the
Small Computer Systems Interface (SCSI).
The SCSI provides
a
powerful general purpose device-independent connection usable by
a wide range of computing systems.
The
ACB-550~
provides
all
of the data separation circuitry
required to convert 5 Mb/sec NRZ to MFM encoded data.
Adaptec's
AIC-250 encode/decode chip designed with a discrete VCO circuit
control write precompensation, addresses mark detect and data
separation functions.
The
ACB-5500
provides
all
required
formatting
and
data
synchronizing functions for ST506 compatible disk drives.
The
formatting
function,
data serializing/deserializing and ECC
checking are provided by Adaptec's proprietary sequencer chip,
the AIC-010.
The ACB-5500 provides up to 2K bytes of dual-ported buffering to
allow high performance data access even if the attached host
system can only accept data at very low rates.
Adaptec's AIC-300
buffer controller provides full dual porting for
the buffer
memory.
The data transfer path is fully checked, using ECC and buffer
parity checking to assure data integrity.
Data integrity is not
compromised by the unchecked control microprocessor.
All low-speed control operations are managed by a powerful 8-bit
microprocessor executing instructions from a 16K read-only con-
trol memory.
The large control memory allows the implementation
of several optional functions as well as a diagnostic self-tst
capability.
The formatting process stores device dependent parameter informa-
tion on the attached disk drives.
When the ACB-5500 is powered
up,
it automatically configures all internal tables from the
stored parameters so that no drive configuration commands are
required from the host system.
Figure 2-1
shows the ACB-5500 block diagram, and Figure 2-2
shows a flow chart of the ACB-5500 microcode structure.
500504-00
2-1
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