5 Dec
2010
5 Dec
'10
3:43 a.m.
On 12/04/2010 03:50 PM, Alexandre Souza - Listas wrote:
Assumes the full speed was used. Often not most of the SCSI to MFM
controllers
I have (adaptex, WD, Xybec) were limited to what the drives could do and
thats
maybe 5MB peak but sustained more like 400kbyts/S. Considering all of them
run 4mhz z80s even thats likely to be unlikely.
I have an AIC4070 Adaptec SCSI to MFM card loose. The core is a 8085
running
3mhz (6mhz crystal) and the memory is a mere 256bytes (8156= 256ram,
22-IO pins
and timer) and the EProm a 2764. There are two asics one to do part of
the MFM disk
interface the other looks to be part of the MFM plus some added IO
function. a pair of
2148s supplied and additional 2k of ram likely as a sector buffer.
An 8085 would be hard pressed to do about 85kbytes second without a DMA
assist.
The DMA Bus speed being processor speed that put the bus in the 3mbyte/S
range maximum.
None of the bridge boards or early (sub 150mb) SCSI disks ran at SCSI
bus speeds.
However the design does show that the cpu and core facilities need not
be big/fast.
The Xybec is a 4mhz Z80, 2116, 2764 and 5380(scsi) and WD asics for MFM
disk.
I pulled these as reference designs.
Drives that are of the era arfe using tms320 and 80c196 cpus (1gb
baracuda class)
and the buffers are in the 256K but that includes LRU cache and cylinder
buffering.
And older RZ23 had a 68hc11,27256 and a 6165 (8k ram) plus a 256Kx8 dram
for
cache and buffer. The basic CPU can be a bench mark for performance.
So those numbers are more about what the BUS was spec'ed to do and not
what actual
devices did.
If reading
speed was an issue, one could employ the faster 4-bit
access to SD rather than the simpler and much slower SPI mode that's
usually used for slow embedded applications.
Hey people, c'mon?!
SD Card speeds:
---8<---cut here---8<---
Speed Class Rating
The Speed Class Rating is the official unit of speed measurement for
SD Cards, defined by the SD Association. The Class number represents a
multiple of 8 Mb/s (1 MB/s), and meets the least sustained write
speeds for a card in a fragmented state.[12]
These are the ratings of some currently available cards:[12]
a.. Class 0 cards do not specify performance, which includes all
legacy cards prior to class specifications.
b.. Class 2, 2 MB/s, slowest for SDHC cards.
c.. Class 4, 4 MB/s.
d.. Class 6, 6 MB/s.
e.. Class 10, 10 MB/s.
---8<---cut here---8<---
Ok, now to SCSI specs
---8<---cut here---8<---
SCSI-1: 5MB/s Fast SCSI: 10MB/s
Fast-Wide SCSI: 10MB/s
Ultra SCSI: 20MB/s
Ultra-Wide SCSI: 40MB/s
---8<---cut here---8<---
So...
- I hope I can develop a SCSI bridge that can move 10MB/s, because
I doubt a normal Atmel XMega will be capable of that.
The atmega might if you go for te fastest part and write tight code.
the problem is it will be waiting for the
SD as they are generally not that fast (it would have to be class 6).
- If I can design a fast enough circuit, I can use
a Class 10 card
and will have 10MB/s.
Why not CF, they are faster. and also IDE can be used.
- Since it will be a hard disk emulator, and not a SD Card reader,
I can have TWO interleaved cards on the bridge, so making 20MB/s easy
Not so fast. SD cards are not like rams/roms and running them
interleaved is going to take a
lot to sync them. It's also a storage management problem. The overhead
will assure that
you get less than twice.
- We are talking old computers here. What do you
use about CLASSIC
computing that goes beyond Fast-Wide SCSI?
- Not all people are capable of begin big.
- Maybe with ARM microcontrollers, Ultra2Wide can be achived. Who
knows?
Time to define DOWN TO EARTH specs and begin working :)
If one is deperate for a drive to run a system then modest speeds that
are in the z80/6mhz
range and SD class 2 are more likely and probably faster on average than
most under
200mb drives were. To replace a 1.07gb Baracuda, thin more like a a CPU
that can do
multiple MIPS of IO performance or faster. The best a Z80 can do with
DMA is 4mbytes/S
and a 8085AH with DMA (8237H) is around 5mb/S. To faster than
SCSI-2your need a
100mhz ARM DMA support (if not on the ARM) and lots of fast everything.
FYI a quick check found plenty of Seagate Barcudas in the 1gb range.
Not cheap but
not scarce either. DEC RZ series drives were easy to find too. If you
looking at fast wide
or faster those are still available. It doesnt look worth the effort at
this time to do faster
than base SCSI.
Allison