Theory Of Operation; Cs80/Esdi Controller [Cec] Pca-A2 - HP 7957A Hardware Manual

Theory Of Operation
3-1. INTRODUCTION
The HP 7957 and HP 7958 Disc Drives are random
access, mass storage devices intended for use with
small and medium size computers. The formatted
capacities of the HP 7957 and HP 7958 are 81
megabytes and 130 megabytes, respectively. In this
chapter "disc drives" refers to both the HP 7957
and HP 7958, unless otherwise specified. The disc
drive has three functional areas divided into three
assemblies (see figure 3-5). The three assemblies are:
the CS80/ESDI Controller [CEC] PCA-A2, the Disc
Drive Assembly [HDA] PCA-Al, and the Power
Supply PCA··A3.
The CEC (PCA-A2) provides CS/80 interface to
the host and Enhanced Small Device Interface
(ESDI) to the HDA. The CEC converts CS/80
commands to ESDI commands and vice versa. It
also provides for error logging to the maintenance
track. The CEC also initiates retries and error cor-
rection. It manages data transfers between the host
and HDA and passes and executes host commands.
NOTE
The CEC also provides for error logging
via an Electrical Erasable PROM
(EEPROM). Instead of logging errors on
the maintenance track these disc drives
log errors (run time and faults) to the
EEPROM.
The HDA (PCA-AI) contains a sealed disc drive
assembly which contains a sealed head-media
module, a preamp PCA, a motor control PCA and
a device electronics PCA. The HDA provides the
means of storing data, converting NRZ code to
Run Length Limited (2,7 RLL) code, write fault
detection, motor control and braking.
The head-disc module in the HDA is sealed and
uses three (HP 7957A) and five (HP 7958A)
130-millimetre (5.12-inch) diameter nonremovable
discs. One read/write head is used for each disc
surface. The bottom surface of the lowest disc in
the stack contains continuous prerecorded servo
data which is used to ensure the precise positioning
of the read/write heads.
3
The power supply (PCA-A3) provides regulated
+5V and + 12V to the CEC and HDA.
Two LED's (red and green) on the front panel
provide information on the disc drive status (see
figure 4-6).
3-2. CS80/ESDI CONTROLLER [CEC]
PCA-A2
The three major functional areas of the CEC (see
figure 3-5) are the HP-IB Interface IC, the Disc
Data Controller (DDC) IC and the microprocessor
(lJP) IC. There are two major buses in the CEC;
the Direct Memory Access (DMA) bus and the lJP
Bus.
The DMA bus is divided into two buses; upper
DMA bus and the lower DMA bus. This is to allow
both 8-bit and 16-bit transfers. The HP-IB
Interface IC is an 8-bit device and the DDC is a
16-bit device.
The EOI detect block looks for the byte tagged
EOI and sends a signal to the DMA Bus Arbiter.
The DMA Bus Arbiter, which controls all access to
the DMA bus, inhibits any further transfer of in-
formation from the HP-IB Interface IC and the
buffers.
Buffer 0 and Buffer 1 are 8k by 8 RAM's.
Information is passed between the HP-IB Interface
IC and the buffers 8-bits at a time.
All odd bytes are transfer to Buffer 1 and all even
bytes are transferred to Buffer O. Transceiver I,
between the DMA bus and Buffer I, is controlled
by the DMA Bus Arbiter, and is turned off for all
even byte transfers. Information passed between
the buffers and DDC is 16-bits at a time and both
buffers are active at this time.
The DDC provides the ESDI Interface to the
HDA. It has an internal programmable error cor-
rection code (ECC) circuit. It also provides
serialization/deserialization of all data and per-
forms all internal control and timing.
3-1