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From: gwyn@smoke.BRL.MIL (Doug Gwyn)
Newsgroups: comp.sys.apple2
Subject: Re: SCSI Terminator
Message-ID: <13644@smoke.BRL.MIL>
Date: 22 Aug 90 20:33:31 GMT
References: <47740433B95FE00E21@UALR.BITNET>
Organization: U.S. Army Ballistic Research Laboratory, APG, MD.
Lines: 32

In article <47740433B95FE00E21@UALR.BITNET> RXBROWN@UALR.BITNET ("MR.FANTASTIC") writes:
>I thought Apple 2s did not need a terminator. I have an 80 Meg drive from
>Tulin and it does not have a terminator. Maybe its built in, but I don't know.
>I just plugged it in and it worked. :)

You were extremely lucky.

All SCSI busses should have termination on both ends.  Improperly
terminated SCSI busses may or may not be usable, but in any event
the bus signal lines would be noisier than intended, due to pulse
echoes off the unterminated end of the bus.  Operating with
reduced margins like this is just asking for trouble.

The original Apple II (non-DMA) SCSI card did not include on-board
termination, but rather relied on a terminator being provided
externally as near to the card as possible (i.e. at the end of the
"SCSI System Cable").  A second terminator would then be needed at
the other end of the bus, unless there was no further bus (i.e.
only one device were attached).

The situation is simpler with the new Apple II (DMA) SCSI card;
since it includes termination, all bus configurations now call for
just one terminator at the far (non-computer) end of the bus.

Most bare SCSI devices are shipped with termination installed;
this should be removed for all devices other than those at the
ends of the bus.  Apple recommends that ALL external SCSI devices
not contain internal termination, so that it is not necessary to
open up the device to toggle its termination.  If no external
device has its own termination, then external terminators such as
the ones that Apple sells are necesary (just one external terminator,
except when using the older SCSI card with multiple external devices).