Nortel DMS-10 Series Installation, Operation And Maintenance Manual page 52

2-24 Section 060-INDP-111 System Description
RAM (DPR). It is considered a peripheral of the microprocessor on the main
CPU (A1).
The SCSI Interface PCA is the link to the disk drive. It transfers data from
and to the bus used for recording on or reading from the disk. The stored
program on the SCSI Interface PCA provides read and write commands to the
disk drive.
This PCA performs a SCSI Host Adapter function and resembles an Initiator
of Priority 7 to the SCSI bus. Target operation is not provided due to the
complexity of an external Y-type crossover network which supports
redundant storage elements. 32 Kbytes of DPR are used as interface and
control for commands and data to and from the MP at slot 1. The SCSI
Interface PCA provides the following capabilities:
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System interface is via the 100-pin edge connector. The EPROM at U16 is a
27C1001 type.
297-6201-502 Standard 01.02 September 2000
Interface between BMC system backplane and SCSI hard disk drives.
32 KBytes of parity protected DPR for command and data passing to and
from the Main CPU at slot 1.
Fully transparent DPR implementation allowing virtually simultaneous
access from either port without using wait states.
Host port of DPR (Main CPU side; slot 1) designed to support fully
asynchronous access based on 4 mHz timing of standard Z80 bus cycle
without WAIT states.
6 MHz processor clock with DPR synchronized and running alternate half
clock cycles. (Host cycles occur during the first half of every T state
when the clock is high, while SCSI side accesses occur during the second
half of each T state when the clock is low).
NCR53C90 SCSI Bus Controller running at 10 mHz. Cycle-Steal DMA
yields approximately 0.4 Mbytes per second across the SCSI bus.
32 KBytes of data RAM with plug-in slots for up to an additional 256
Kbytes.
Up to 64 KBytes of program PROM.
Support for disconnect/reconnect operation on the SCSI bus.
Single error detection parity guard on DPR memory.
50 pin access port for logic analyzer, emulator and test support (factory
use only).