Polaroid Sprintscan and SCSI

[jojax14]

Hi all,

Based on a suggestion from members of comp.periphs.scanners, I have
purchased a Polaroid Sprintscan scanner on eBay. This will allow me to
scan some 126 slides that I have without having to spend a fortune on
a medium format scanner, as no other 35mm scanners will take the full
height of the slide.

The scanner arrived without a SCSI cable. There are two SCSI ports on
the back - a DB25 and a centronics 50 pin. I already have a cable
that would connect my PC to the scanner (from an old SCSI Zip drive)
using the DB25 connector, but the manual only ever gives examples of
using the Centronics port for connection to the computer, with the
DB25 port used for connections to subsequent devices. However, the
manual states "A SCSI cable can be connected to either port without
affecting the performance of your equipment", but this is ambiguous,
as it could just mean that the scanner will not run slower if there
are other devices on the SCSI chain.

Is SCSI a bus-type structure, where I can use either of the available
ports, or is there a definite 'in' and 'out'?

If the answer to that question is 'yes' and I can use the cable, there
is then the issue of termination. The scanner has a built-in
terminator. Would the internal terminator work for both ports, or
only for the centronics port?

Thanks in advance,

Jojax14

 

[Brooks Moses]

The scanner arrived without a SCSI cable. There are two SCSI ports on
the back - a DB25 and a centronics 50 pin. I already have a cable
that would connect my PC to the scanner (from an old SCSI Zip drive)
using the DB25 connector, but the manual only ever gives examples of
using the Centronics port for connection to the computer, with the
DB25 port used for connections to subsequent devices. However, the
manual states "A SCSI cable can be connected to either port without
affecting the performance of your equipment", but this is ambiguous,
as it could just mean that the scanner will not run slower if there
are other devices on the SCSI chain.

Is SCSI a bus-type structure, where I can use either of the available
ports, or is there a definite 'in' and 'out'?

It is a bus-type structure. (As I understand it -- though I'm sure
comp.periphs.scsi regulars will correct me -- it's also nominally a bus
design that has more than 25 wires in it, so a 25-pin connector and
cable is a bit of a kludgey compromise, but it works ok for one or two
devices.)

I've never seen a DB25-to-DB25 SCSI cable; are you sure that this wasn't
a parallel-port Zip drive?

Worst case is that you hook it up and it doesn't work; I don't think
you're likely to damage anything if it doesn't work. Ergo, my
suggestion would be test it and see, and consider that your definitive
answer.

If the answer to that question is 'yes' and I can use the cable, there
is then the issue of termination. The scanner has a built-in
terminator. Would the internal terminator work for both ports, or
only for the centronics port?

I would think it would work for either port.

- Brooks

 

[Eastside]

The scanner arrived without a SCSI cable. There are two SCSI ports on

the back - a DB25 and a centronics 50 pin. I already have a cable
that would connect my PC to the scanner (from an old SCSI Zip drive)
using the DB25 connector, but the manual only ever gives examples of
using the Centronics port for connection to the computer, with the
DB25 port used for connections to subsequent devices. However, the
manual states "A SCSI cable can be connected to either port without
affecting the performance of your equipment", but this is ambiguous,
as it could just mean that the scanner will not run slower if there
are other devices on the SCSI chain.

SCSI supports a daisy chain topology. You should be able to use either as
in or out although I've always seen either 2 Centronics or 2 DB25 ports,
never one of each. I think what to Polaroid was trying to indicate is that
you use sets of DB25-Centronics cables to daisy chain with the DB25
connected to the device closest to the computer and the Centronics connected
to the next device.

Is SCSI a bus-type structure, where I can use either of the available
ports, or is there a definite 'in' and 'out'?

If the answer to that question is 'yes' and I can use the cable, there
is then the issue of termination. The scanner has a built-in
terminator. Would the internal terminator work for both ports, or
only for the centronics port?
Jojax14

Again, shouldn't matter.

Dane

 

[Nico de Jong]

Again, shouldn't matter.

I humbly dont agree. When the scanner terminates the cable, this must mean
that the termination is "between" to two connectors.
This again means, that you can use either one (but not both) connectors, as
you otherwise would have a termination in the middle of a chain, and that is
absolutely forbidden. In other words : the scanner must always be the last
device on the chain (unless you can disable the terminator)

Nico

 

[Brooks Moses]

I humbly dont agree. When the scanner terminates the cable, this must mean
that the termination is "between" to two connectors.
This again means, that you can use either one (but not both) connectors, as
you otherwise would have a termination in the middle of a chain, and that is
absolutely forbidden. In other words : the scanner must always be the last
device on the chain (unless you can disable the terminator)

How does a pass-thru terminator work, then?

- Brooks

 

[Nico de Jong]

How does a pass-thru terminator work, then?

- Brooks

You got me there. For me, not being a native english speaker, it is a
contradiction in terms. "Terminate" says in my dictionary "Stop here". "Pass
through" says "dont stop". In other words, "you must stop here, but you can
pass through" .
Would you care to explain please ?

Nico

 

[Brooks Moses]

You got me there. For me, not being a native english speaker, it is a
contradiction in terms. "Terminate" says in my dictionary "Stop here". "Pass
through" says "dont stop". In other words, "you must stop here, but you can
pass through" .
Would you care to explain please ?

There's this little, inch-long piece of plastic with a male
Centronics-50 plug on one side and a female on the other, which goes
between the end of a SCSI cable and the device it's plugged into. When
it's connected in that manner, the SCSI chain is properly terminated,
and the device it's plugged into is the last device on the chain. This
device was in a bin at the parts store labeled "pass-through
terminator".

That's all I know.

In any case, if your previous advice (about the terminator only working
for one plug) is based only on the dictionary definition of "terminate"
rather than a direct knowledge of how SCSI connections work, it's
essentially useless speculation. Technical jargon, such as "SCSI
terminator", rarely has the same definition as the same word does in a
non-technical context -- essentially, "SCSI terminator" is the best word
that the SCSI engineers could come up with for "this thing that goes on
or near the end of the SCSI chain to prevent the SCSI bus from having
electrical noise resonances"; that doesn't mean it actually "terminates"
in all senses of the word. And so it's not surprising that there are
technical phrases that look like contradictions in terms; they _are_
contradictions in terms if you look at the terms too closely.

- Brooks

 

[Wayne Fulton]

You got me there. For me, not being a native english speaker, it is a
contradiction in terms. "Terminate" says in my dictionary "Stop here". "Pass
through" says "dont stop". In other words, "you must stop here, but you can
pass through" .
Would you care to explain please ?

The pass-through SCSI terminator is a housing to provide resistors on each
active wire, a 220 ohm resistor up to +5V, and a 330 ohm resistor down to
ground, which stablizes the voltage and is an impedance matching load on the
line to terminate signal reflections from the open end. It doesnt matter if
this is slightly outside the connector (pass through), or slightly inside the
connector (internal). The internal termination is probably active
termination today, same thing but using transistors as an active voltage
regulator.

The SCSI bus just flows though the SCSI device, an unbroken path, in one
connector and out the other, to allow a daisy chain of devices on the bus.
It doesnt matter which of the two connectors you use, only matters if the
connector pins fit.

 

[Folkert Rienstra]

SCSI supports a daisy chain topology. You should be able to use either as
in or out although I've always seen either 2 Centronics or 2 DB25 ports,
never one of each. I think what to Polaroid was trying to indicate is that
you use sets of DB25-Centronics cables to daisy chain with the DB25
connected to the device closest to the computer and the Centronics connected
to the next device.


Again, shouldn't matter.

Depends on whether it is manual or automatic (with one of the
connectors being DB25, it is probably manual).
If it is automatic the mechanism may be tied to one of the two
connectors and one would be designated "in" and the other "out".
In such case when the out connector is used the internal terminator disables.

 

[Folkert Rienstra]

I humbly dont agree.

When the scanner terminates the cable,

Now reread that carefully.

this must mean that the termination is "between" to two connectors.

But still terminates "the cable".

This again means, that you can use either one (but not both) connectors,
as you otherwise would have a termination in the middle of a chain,

So obviously it doesn't work that way.

and that is absolutely forbidden.
In other words : the scanner must always be the last device on the chain

Not if it has an in and out connector (i.e. is daisy chainable).

(unless you can disable the terminator).

Bingo.
On devices with in and out connectors that usually is the case,
certainly on a professional scanner.

 

[jojax14]

I tried out the DB25 adaptor, and it worked perfectly. The scanner's
internal terminator does seem to be doing it's job, as I have had no
communication problems with the scanner over a couple of hours use.

Just out of interest, according to the manual for the scanner, the LE &
ES versions of the Polaroid Sprintscan require an external terminator.
The diagram in the manual shows that you plug the terminator into the
scanner, and then the cable to the PC goes into the terminator. This
means that the terminator is in fact in-line.

I did some more reading after my initial post yesterday, and the main
purpose of a terminator is to stop signal reflections, through the use
of resistors. It does not complete a circuit like I had initially
thought. This could explain why a terminator can work when it is
in-line.

Thanks for all the help,

Jojax14

 

[Nico de Jong]

There's this little, inch-long piece of plastic with a male
Centronics-50 plug on one side and a female on the other, which goes
between the end of a SCSI cable and the device it's plugged into. When
it's connected in that manner, the SCSI chain is properly terminated,
and the device it's plugged into is the last device on the chain. This
device was in a bin at the parts store labeled "pass-through
terminator".

---
Ah, nice to know that such a thing exists.

In any case, if your previous advice (about the terminator only working
for one plug)

--
Thats not what I said (or at least : didnt mean)
I've been in SCSI since 1986, so I'm not a complete novice.
From what I got out of the original description, the box contained connector
<> terminated drive <> connector.
What I meant, was that you could not have other drives in after the
terminated drive, as seen from the adapter. That would mean either two
terminations, or a drive _after_ the termination, which is not allowed.
To put things straight for those who are confused : terminators are only to
be used at both ends.
Nico