At 10:08 10/29/99 , [log in to unmask] wrote:
 > [My] ACER 310S Scanner...came with an ISA SCSI Card which is
 > working well. I recently purchased an Adaptec 2906 PCI SCSI
 > Card... The scanner...did not come with a terminator and does
 > not have one installed.
 >
 > While looking up the price of a terminator, I discovered a
 > wide selection...My question is, what type of terminator
 > should I get. The scanner is my only external device.



Hi Chuck

The scanner probably came without a terminator because to save
a few dollars. Inexpensive scanners transfer data so slowly that
they can get away without terminating the SCSI bus. If you have
any other SCSI devices on the SCSI bus, you absolutely will need
to terminate the external end of the SCSI chain. (Also, the
Adaptec card may require the bus terminated at the scanner even
if the scanner is the only device.)

If you are ever going to use any other devices on this SCSI
bus, you should get an ACTIVE terminator. (PASSIVE terminators
are cheaper but one would suffice only if the scanner is the only
device attached to the SCSI controller or if nothing faster than
an older CDROM drive were present.)

If the scanner doesn't have two SCSI sockets (you are using
one for the external SCSI cable and you need another to plug
in a terminator), you will need either a "pass through" terminator
module (this is a terminator module with two plugs...one would
plug into the scanner's socket and you would plug your external
SCSI cable into the other side of the module) or a "T" adaptor
*and* a seperate terminator module. The "T" adaptor would
plug into the scanner and provide two empty sockets (one for
plugging in the terminator and one for the SCSI cable).

Once you have determined what kind of terminator you need, you
have to worry about what kind of connector your scanner has.
Is it a 25 pin "Apple" type or a 50 pin "Centronics" type (25
extra "ground" wires between the signal wires) or a 50 pin
"High Density" (sometimes called Micro-D) connector? See this
link for pictures: <http://www.connectronics.com/scsi.htm>
Pay attention also to the gender of all your connectors.

So you need to get an *active* terminator module (pass through or
not and if necessary a separate "T" adaptor) that will fit the
connectors (socket on your scanner) and the connector on your
external cable. Of course if your scanner does have two SCSI
connectors (sockets), you don't need to worry about getting a
pass through type of terminator or (alternatively) a separate
"T" adaptor. (In case you're wondering how the scanner can work
when it "follows" the terminator, you have to imagine that the
scanner is on a short "stub" going off the end of the SCSI bus
right by the terminator.)

Lastly, consider that your external SCSI cable might not be of
high enough quality to use with other faster SCSI devices on
the SCSI bus. (Whether devices are on an internal or external
cable...or both, they are all on the one SCSI bus.) The cable
might not provide proper insulation from stray external signals
or prevent cross talk between wires inside the cable or it
might cause problems due to signal degradation at it's ends.
And worst of all, some cheap SCSI cables do not even have all
50 wires connected from one end to the other. (For example,
the line that supplies power to the terminator module might not
be present in the cable that came with the scanner.) High
quality SCSI cables can cost $50 or more. Don't expect a $100
scanner (say) to come with a $50 cable. (I'm not familiar with
Acer scanners and their prices.)

Here are some URLs (in no particular order that cover a wide
price range) where you can get terminators and cables:
https://www.hypermicro.com/store/index.htm
http://www.scsi-cables.com/
http://www.scsipro.com/catalog/index.htm
http://www.tmcscsi.com/
http://www.connectronics.com/scsi.htm
http://www.dalco.com/
http://www.cablemakers.com/
http://www.mcpb.com/html/ibmscs2.html
http://www.corpsys.com/
http://www.jdr.com/interact/default.asp
http://www.jameco.com/

Regards,
Bill

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