[rescue] DSBE/S

[Jonathan C. Patschke]

> Differential SCSI/Ethernet.
> 
> This is what I need for the RSM-214, right?

That should work, yep.

> After hitting buy it now, I noticed that the seller describes it as a
> 68pin narrow single ended differential controller.

The seller is an idiot.  Either it's narrow or differential.  Although he
may think that "single-ended" means "has one port", as opposed to PCI
cards, which typically have an internal and external port.

> The associated picture shows the card with the words differential scsi
> above the 68pin plug, but the logo next to the words doesn't look like
> the normal differential logo.

It should be something like:
   / ^
  / / \
 / /   \
< <   ----
 \ \   /
  \ \ /
   \ V

Although I have seen some vendors pull a really stupid stunt of putting
the SE SCSI logo on parts, and prefixing it with "DIFF".

> Seller also says "Part Number(501-1902)/Kit Number X1052A"

According to Google, that'll do just what you want it to.

> So, the handbook says that it is a differential scsi/ethernet card, and not
> much else.
> 
> Now, my understing is that differential is always wide, 

I'll play "professor" today, but I'll try not to be an ass about it.  Feel
free to point out any mistakes.  It's been a while since Elec326.

A lot of people have gone over SCSI lately (and given a hell of a lot of
info that I never knew), but no-one's given an actual explanation about
what "differential" means.  That might be what's tripping you up.

Differential interfaces can run either wide or narrow SCSI.  It's
completely independent of the type of SCSI run over the interface.

Think of "single-ended" and "differential" as two completely different
types of physical interfaces (because they are), like Ethernet versus
Token Ring.  Just like you can run NetBEUI, IP, or IPX over either, you
can run most-any variant of SCSI over either bus type.  Also, just like
Token Ring and Ethernet, both SE and DIFF can be throttled to different
speeds, so long as all the devices on the bus support it.

As far as actually what differential signalling is, here's a brief
explanation.  Imagine that you're running a single wire (like, a single
copper strand from a cat-5 cable) across a noisy machine room to connect
two computers with a one bit serial interface[1].  You're using fairly
simple signalling:  4-5 volts is a "1", 0-1 volts is a "0".

Now, imagine that there's some electromagnetic noise (say, from a big
power supply in a nearby E6500) in the room that causes some inductance in
the cable, fluctuating the voltage a bit.  All of a sudden, that 0-1 and
4-5 volt range of binary logic is getting readings in the 2-3 and 6-7
scale, totally confusing the computers that are trying to hold a decent
conversation.

Differential signalling is a pretty decent solution to this problem.  To
modify the serial interface for differential signalling, run a second wire
between the two computers, and tie them together.  Always send the
-opposite- signal along the second wire.  That is:  When you're trying to
send a "1", send a "1" on the first line, and a "0" on the other.

The theory is that any EM noise will be localized enough to affect both
wires in pretty-much the same way.  So, at any point in time, if line 1
induces an extra volt of potential, line 2 probably will, too.

To decode the information on the other end, measure the potential
difference across the two wires (to determine whether the difference is
large enough for the signal to still be good--in this case it should
always be +/- (3-5 volts)), and determine a "0" if line 2 has higher
voltage, or "1" if line 1 has higher voltage.  Noise problem solved.

> and differential and single ended are mutually exclusive.  Did I just
> do something stupid?

Nope, you bought exactly what you needed to.  That particular card is a
bit slow, but it makes up for that by giving you a second Ethernet port.

--Jonathan
[1] For the moment, ignore the details of handshaking and the other nitty-
    gritties of serial communication.