3 Sep
2006
3 Sep
'06
12:08 p.m.
Hi,
I want to move my scanners over to one of these Macs
(probably the 840AV since I can also exploit the video
capabilities thereof).
But, it's got those wacky DB25 SCSI connectors. :-/
I have DB25-Centronics cables. And, DB25-HD50 adapters.
The scanners have a hodge-podge of *different* connectors
(sheesh! talk about "standards"... :< ).
I assume a good goal is to minimize *connections* wherever
possible. So, instead of DB25-HD50 adapter to HD50 cable,
use DB25-HD50 cable, if possible.
But, I'm nervous about the DB25's and their roles in all this.
Is this just a regular SCSI (narrow, SE) bus with a few less
returns? Or, are there other consequences of this connector
choice?
E.g., how much grief can I expect going *through* the slide
scanner with DB25 coming in and Centronics going out? Is
this just effectively a "DB25-Centronics" adapter with a
device tap in the middle?
Thanks!
--don
3 Sep
3 Sep
1:14 p.m.
Don wrote:
Hi,
I want to move my scanners over to one of these Macs
(probably the 840AV since I can also exploit the video
capabilities thereof).
But, it's got those wacky DB25 SCSI connectors. :-/
I have DB25-Centronics cables. And, DB25-HD50 adapters.
The scanners have a hodge-podge of *different* connectors
(sheesh! talk about "standards"... :< ).
I assume a good goal is to minimize *connections* wherever
possible. So, instead of DB25-HD50 adapter to HD50 cable,
use DB25-HD50 cable, if possible.
But, I'm nervous about the DB25's and their roles in all this.
Is this just a regular SCSI (narrow, SE) bus with a few less
returns? Or, are there other consequences of this connector
choice?
E.g., how much grief can I expect going *through* the slide
scanner with DB25 coming in and Centronics going out? Is
this just effectively a "DB25-Centronics" adapter with a
device tap in the middle?
Thanks!
--don
The main thing to worry about when the DB25 is involved
is that the cable that is used is scsi certified. The round cable
that is usually used between the two connectors if it is there
has a special way of positioning the req and ack signals
in the bundle of wires. Other than that, the standard specified
the connector, so that should be no worry.
I know early on, some cables were made and just used that
may not have been SCSI certified, and in early "vintage" systems
you may run across such, and have noise or performance
problems with the devices, if the REQ / ACK / Select signals
get messed up.
Only when you go from 50 -> 68 pin should you beware of
termination on the upper 8 bits added by the extra lines and
what you have for an initiator and targets.
There is not much science in a wide initiator selecting narrow
devices on a chain, but there is when you have wide targets and
a narrow initiator.
but you don't have that from what I read, so it is just a matter
of haveing a bit of everything in the chain connetorwise, but
the SCSI bus should all be the same.
1:48 p.m.
On Sunday 03 September 2006 04:14 pm, jim stephens wrote:
Only when you go from 50 -> 68 pin should you
beware of
termination on the upper 8 bits added by the extra lines and
what you have for an initiator and targets.
There is not much science in a wide initiator selecting narrow
devices on a chain, but there is when you have wide targets and
a narrow initiator.
What sort of problems would you expect doing this?
My first SCSI setup some years back involved a wide drive with an adapter, a
50-wire ribbon cable, and an Adaptec 2842 which worked fine until a storm
rolling through here took out the drive, the 2842, and the MB. :-(
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
10:27 p.m.
Roy J. Tellason wrote:
On Sunday 03 September 2006 04:14 pm, jim stephens
wrote:
SCSI Single ended external case from main system. They didn't have
the HVD to cover the noise problems.
I am a long time away from the arguments, but I understand that
the problem was with mixing the pulses in the REQ and ACK in
when one was in the selection phase, and getting bad target
selects, by ringing on the data lines while doing selects. So the
lines with the sharp rise times were put in the middle of the bundle
and wrapped with grounds to keep that from happening..
There is a certified SCSI round cable especially for the round
type cables, like you usually have with DB25 type cables.
It isn't the same as you would use with RS232 or other types
of 25 pin full cables.
Jim
<snip>
What sort of problems would you expect doing this?
11:42 p.m.
Roy J. Tellason wrote:
On Sunday 03 September 2006 04:14 pm, jim stephens
wrote:
Since SCSI devices are selected via individual data bits,
a wide device having an ID > 7 would be selected via DATAn
(where n > 7). A narrow device (initiator) doesn't *have*
those data lines connected to it (hence no way for it to
select a device > 7)
Only when you go from 50 -> 68 pin should you
beware of
termination on the upper 8 bits added by the extra lines and
what you have for an initiator and targets.
There is not much science in a wide initiator selecting narrow
devices on a chain, but there is when you have wide targets and
a narrow initiator.
What sort of problems would you expect doing this? My first SCSI setup some years back involved a wide
drive with an adapter, a
50-wire ribbon cable, and an Adaptec 2842 which worked fine until a storm
rolling through here took out the drive, the 2842, and the MB. :-(
11:48 p.m.
jim stephens wrote:
Yes, I can recall the cross-sectional illustration of the
cable (I have the SCSI-2 spec here along with ENDL's bench
reference...). No, Jim, I wouldn't be inclined to use an
RS232 cable to connect the two DB25's! :>
But, the only way I can *guess* that a cable is "suitably
designed" is to *infer* that since it has, e.g., DB25 on one
end and Centronics50 on the other, it *seems* likely that
this was it's intended purpose (though who knows how much crap
was spilled onto the Mac market, etc.)
Don wrote:
> But, it's got those wacky DB25 SCSI connectors. :-/
>
> I have DB25-Centronics cables. And, DB25-HD50 adapters.
> The scanners have a hodge-podge of *different* connectors
> (sheesh! talk about "standards"... :< ).
E.g., how much
grief can I expect going *through* the slide
scanner with DB25 coming in and Centronics going out? Is
this just effectively a "DB25-Centronics" adapter with a
device tap in the middle?
The main thing to worry about when the DB25 is involved
is that the cable that is used is scsi certified. The round cable
that is usually used between the two connectors if it is there
has a special way of positioning the req and ack signals in the bundle of wires. Other than that, the standard
specified
the connector, so that should be no worry.
I know early on, some cables were made and just used that
may not have been SCSI certified, and in early "vintage" systems
you may run across such, and have noise or performance
problems with the devices, if the REQ / ACK / Select signals
get messed up.
Only when you go from 50 -> 68 pin should you beware of
termination on the upper 8 bits added by the extra lines and
what you have for an initiator and targets.
Yeah, no problem there.
There is not much science in a wide initiator
selecting narrow
devices on a chain, but there is when you have wide targets and
a narrow initiator.
but you don't have that from what I read, so it is just a matter
of haveing a bit of everything in the chain connetorwise, but
the SCSI bus should all be the same.
Thanks! I guess I'll just do the "cross fingers and hope" trick.
At least any problems will be more "visible" (and, less *hazardous*
than if the devices were disks, etc.).
4 Sep
4 Sep
2:10 p.m.
On Monday 04 September 2006 02:48 am, Don wrote:
Any reason a 25-wire _ribbon_ cable wouldn't be useful for this?
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
The main thing
to worry about when the DB25 is involved
is that the cable that is used is scsi certified. The round cable
that is usually used between the two connectors if it is there
has a special way of positioning the req and ack signals
Yes, I can recall the cross-sectional illustration of the
cable (I have the SCSI-2 spec here along with ENDL's bench
reference...). No, Jim, I wouldn't be inclined to use an
RS232 cable to connect the two DB25's! :>
3 Sep
3 Sep
1:41 p.m.
On Sunday 03 September 2006 03:08 pm, Don wrote:
I assume a good goal is to minimize *connections*
wherever
possible.
Why?
So, instead of DB25-HD50 adapter to HD50 cable, use
DB25-HD50 cable, if
possible.
The main difference would be way fewer ground wires. If the 50-wire cable is
ribbon cable then every other wire is a ground, which would tend to minimize
interactions betweeen adjacent signals. If it's not ribbon cable I'm not
sure how they do it. I would imagine that a 25-pin connector would give you
lower signal quality overall.
But, I'm nervous about the DB25's and their
roles in all this.
Is this just a regular SCSI (narrow, SE) bus with a few less
returns? Or, are there other consequences of this connector
choice?
See above. Given the choice to use it or not I'd opt not to, myself. But
the exact results are going to depend on a lot of factors like the quality of
the cable, the amount of electrical noise in your environment, the quality
of what's driving both ends, etc.
E.g., how much grief can I expect going *through* the
slide
scanner with DB25 coming in and Centronics going out?
I'm not sure that you even _can_ do this without seeing some detailed
technical info on the scanner.
Is this just effectively a "DB25-Centronics"
adapter with a device tap in
the middle?
No way to tell without looking...
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
11:40 p.m.
Roy J. Tellason wrote:
Scanner has DB25 and Centronics connectors. Obviously, an attempt
to be compatible with a variety of different applications.
They are no doubt wired AS IF they were two "identical" connectors
with the scanner itself sitting *logically* between them (though
I have seen this PHYSICALLY violated so often on SCSI devices
that I wouldn't make any bets on it!).
So, it *suggests* running *into* the slide scanner on one connector
and *out* (to the next downstream device) on the *other* connector.
(scanners often only have a single connector which makes putting
more than one on a given SCSI chain a bit of a chore)
On Sunday 03 September 2006 03:08 pm, Don wrote:
- connections give rise to *broken* connections (i.e. cable
gets bumped, etc.)
- connectors almost invariably add C
- connectors often introduce impedance mismatches (not from
the connector, per se, but what's behind it)
I assume a good goal is to minimize *connections*
wherever
possible.
Why? E.g., how much
grief can I expect going *through* the slide
scanner with DB25 coming in and Centronics going out?
I'm not sure that you even _can_ do this without seeing some detailed
technical info on the scanner. Is this just
effectively a "DB25-Centronics" adapter with a device tap in
the middle?
No way to tell without looking... 
4 Sep
4 Sep
11:30 a.m.
On Sun, 3 Sep 2006, Don wrote:
Scanner has DB25 and Centronics connectors. Obviously, an attempt
to be compatible with a variety of different applications.
They are no doubt wired AS IF they were two "identical" connectors
with the scanner itself sitting *logically* between them (though
I have seen this PHYSICALLY violated so often on SCSI devices
that I wouldn't make any bets on it!).
So, it *suggests* running *into* the slide scanner on one connector
and *out* (to the next downstream device) on the *other* connector.
(scanners often only have a single connector which makes putting
more than one on a given SCSI chain a bit of a chore)
Which won't work unless you can disable termination in the scanner, right?
If the scanner was designed to only ever use one of the connectors (always
be at the end of a chain) and not require external termination, that might
not be possible...
Alexey
E.g., how much grief can I expect going *through* the
slide
scanner with DB25 coming in and Centronics going out?
I'm not sure that you even _can_ do this without seeing some detailed
technical info on the scanner.
11:43 a.m.
Alexey Toptygin wrote:
On Sun, 3 Sep 2006, Don wrote:
Scanner has DB25 and Centronics connectors. Obviously, an attempt
to be compatible with a variety of different applications.
They are no doubt wired AS IF they were two "identical" connectors
with the scanner itself sitting *logically* between them (though
I have seen this PHYSICALLY violated so often on SCSI devices
that I wouldn't make any bets on it!).
So, it *suggests* running *into* the slide scanner on one connector
and *out* (to the next downstream device) on the *other* connector.
(scanners often only have a single connector which makes putting
more than one on a given SCSI chain a bit of a chore)
Which won't work unless you can disable termination in the scanner,
right? If the scanner was designed to only ever use one of the
connectors (always be at the end of a chain) and not require external
termination, that might not be possible...
Scanner does not have an internal terminator. Currently
terminated with a feed-thru Centronics50 terminator on the
*one* connection to the Centronics port.
E.g., how much grief can I expect going *through* the
slide
scanner with DB25 coming in and Centronics going out?
I'm not sure that you even _can_ do this without seeing some detailed
technical info on the scanner. 
2:30 p.m.
Alexey Toptygin wrote:
Which won't work unless you can disable
termination in the scanner,
right? If the scanner was designed to only ever use one of the
connectors (always be at the end of a chain) and not require external
termination, that might not be possible...
Alexey
It really depends on the scanner. My HP 4C has both connectors, one for
input and one for output. A SCSI Zip drive hangs off of it.
I've never seen a SCSI scanner that had two different ports but only let
you use one. Seems contrary to the way SCSI is cabled. If it has two
ports, it probably lets you pass through.
4:24 p.m.
On 9/4/2006 at 4:30 PM Michael B. Brutman wrote:
I've never seen a SCSI scanner that had two
different ports but only let
you use one. Seems contrary to the way SCSI is cabled. If it has two
ports, it probably lets you pass through.
My HP ScanJet C6270 is SCSI and has but a single HD50 connector.
Cheers,
Chuck
4:51 p.m.
Chuck Guzis wrote:
On 9/4/2006 at 4:30 PM Michael B. Brutman wrote:
Scanners vary widely. I have one with DB25&Cent50; another
with HD50&USB; another with just Cent50; another with just
HD50. I know cheapr scanners also exist with parallel
port interfaces, etc.
I've never seen a SCSI scanner that had two
different ports but only let
you use one. Seems contrary to the way SCSI is cabled. If it has two
ports, it probably lets you pass through.
My HP ScanJet C6270 is SCSI and has but a single HD50 connector.
5:37 p.m.
On 9/4/2006 at 4:51 PM Don wrote:
Scanners vary widely. I have one with DB25&Cent50;
another
with HD50&USB; another with just Cent50; another with just
HD50. I know cheapr scanners also exist with parallel
port interfaces, etc.
Yup--the aforementioned ScanJet also has a USB interface.
5:05 p.m.
Chuck Guzis wrote:
On 9/4/2006 at 4:30 PM Michael B. Brutman wrote:
What did I miss here? You C6270 has one port. I was talking about
scanners that have two ..
I've never seen a SCSI scanner that had two
different ports but only let
you use one. Seems contrary to the way SCSI is cabled. If it has two
ports, it probably lets you pass through.
My HP ScanJet C6270 is SCSI and has but a single HD50 connector.
Cheers,
Chuck 
8:38 p.m.
Just as a side note here, IIRC the Apple OneScanners all were spec'd
to be at the end of the SCSI chain.
Jim
2:08 p.m.
On Monday 04 September 2006 02:40 am, Don wrote:
<...>
Scanner has DB25 and Centronics connectors. Obviously, an attempt
to be compatible with a variety of different applications.
They are no doubt wired AS IF they were two "identical" connectors
with the scanner itself sitting *logically* between them (though
I have seen this PHYSICALLY violated so often on SCSI devices
that I wouldn't make any bets on it!).
Well, that's the assumption that I'm wondering about, but then I haven't
gotten into the insides of too many of those units, so I just don't know...
E.g., how much grief can I expect going *through* the
slide
scanner with DB25 coming in and Centronics going out?
I'm not sure that you even _can_ do this without seeing some detailed
technical info on the scanner. So, it *suggests* running *into* the slide scanner on
one connector
and *out* (to the next downstream device) on the *other* connector.
(scanners often only have a single connector which makes putting
more than one on a given SCSI chain a bit of a chore)
I have a couple of external SCSI CDROM drives here that are like that, too.
And have not had much success in finding any sort of a "Y-adapter" or
something I could use to deal with that.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
2:33 p.m.
Roy J. Tellason wrote:
On Monday 04 September 2006 02:40 am, Don wrote:
For Centronics50 connections, there are cables that have
three connectors on them (one on each end plus an additional
one on the BACK of one of the ends). Not an ideal solution
as it can lead to some nasty stub lengths...
So, it *suggests* running *into* the slide
scanner on one connector
and *out* (to the next downstream device) on the *other* connector.
(scanners often only have a single connector which makes putting
more than one on a given SCSI chain a bit of a chore)
I have a couple of external SCSI CDROM drives here that are like that, too.
And have not had much success in finding any sort of a "Y-adapter" or
something I could use to deal with that.
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19 comments
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