SCSI to IDE

[allison]

On 12/04/2010 03:50 PM, Alexandre Souza - Listas wrote:
>> 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

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.

> 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