On Sun, 15 Nov 1998 Uzi Paz <[log in to unmask]> wrote:
>There are quite a lot of standards for SCSI outlets and sockets, there are
>at least two types of 50pin (external), there are 40pin, 25pin, 68pin
>sockets.
'The nice thing about standards is that there are so many to choose from'
and of course, some are proprietary, some ad hoc and some are official.

>I wish to know if there is somewhere how to map between different
>standards, and what do the extra pins mean, i.e. if I use a 50pin to 25pin
>cable what do I lose?
Proper termination - and that's important.

If you use a 25 way connector you don't lose any signal lines but you do
lose some of the paired grounds, and hence lose the ability
to terminate the signal lines properly. You should try to avoid using
the 25 way connector, and if you _have_ to use it, avoid mixing it with
the 50 way connector and keep all cables short.

(Cheap SCSI cables may also economise on ground wires, even if they
have 50 way connectors. Cables may also vary from the standard
characteristic impedance. Hence mixing cables of doubtful origin
can be a source of unreliability.)

>I am now interested in mapping between 50pin old (big socket) standard and
>the 25pin,
>and the mapping between the 68 pin and the new (small socket) 50pin
>standard.
The 25 and 50 way connectors are an 8 bit SCSI bus. The two sizes of 50 way
should be freely interconnectable with appropriate adaptors.
The 68 way is a 16 bit SCSI bus. If you want to connect 8 bit devices
to a 16 bit bus (it is allowed) you must make sure the upper 8 bits
are correctly terminated at the cable convertor.

There is also differential SCSI, used on some high-performance HDs,
which CANNOT be connected to the other kinds.

Joakim Ögren's 'The Hardware Book' lists many connector pinouts and
other useful data, but you will have to search for it because it
vanished from its home site.
When I downloaded it (HWB12HTM.zip, about 800K) there were several
mirror sites.

There are several SCSI FAQs to be found quite easily on the Web.
Some I've found justify keeping on my hard disk are:-

Frequently Asked Questions List for comp.periphs.scsi
http://www.ultranet.com/~gfield/gary/scsifaq.html
(the biggest and best read)

http://support.quantum.com/faq/scsifaq.htm
(HD oriented but short and useful if you know little about SCSI)

http://www.glyphtech.com/glscsifq.htm
Glyph Technologies Inc. SCSI Frequently Asked Questions

http://www.maxcomp.com/tips/fpt.htm
Forced Perfect Termination (FPT)

Make sure you understand passive and active termination.
FPT is a more advanced form of active termination.

Much more of this and I'll be writing a SCSIFAQ FAQ :-)


Len Warner <[log in to unmask]> http://easyweb.easynet.co.uk/~len/ ICQ:10120933

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