dplatt@coherent.com (Dave Platt) (02/22/90)
Original-posting-by: dplatt@coherent.com (Dave Platt) Original-subject: Re: Can the SCSI bus be terminated w/o powering up noisy device? Reposted-by: emv@math.lsa.umich.edu (Edward Vielmetti) In article <17008@boulder.Colorado.EDU> fozzard@boulder.Colorado.EDU (Richard Fozzard) writes: > I recently got a Syquest drive (Ehman) that I only need occasional access > to. Its fan is painfully loud, so I would like to leave it powered down > when not in use. However, I cant do this since my SE/30 with internal > Sony 40 meg wont boot if the SCSI cable is connected, but the external > drive is powered down. Presumably, the termination resistors in the > Ehman require power to work. > > Is there any way to terminate the SCSI bus correctly w/o powering up > that damned noisy Ehman? It does have a 2nd SCSI (25-pin) connector on > the back, and it is the only external SCSI device. Yes, there is a way. It's not all that difficult. 1) Purchase one external SCSI terminator ($20 or so)... these are small blocks (about 1" thick) with a 50-conductor male Centronics plug on one side and a 50-conductor female Centronics jack on the other side. Also purchase one standard "Mac to SCSI peripheral" cable... male DB-25 on one end, and 50-conductor male Centronics plug on the other ($25 or so). 2) Open up the Ehman case. Find the internal termination resistors on the SyQuest controller board (probably 2 or 3 single-in-line or dual-in-line resistor packs plugged into the board near the 50-pin SCSI header). Remove the resistors with a small screwdriver; save for future use. Close the case. 3) Plug the DB-25 plug on the cable into the second 25-pin jack on the back of the Ehman case. Plug the other end of the cable (male) into the female jack on one side of the external terminator. 4) Cover the unused (50-pin male plug) side of the external terminator with black electrical tape, so that you don't short out the bus. Your SE/30 should now be able to boot correctly even if the Ehman is not powered up. The SE/30 provides terminator power (as do the SE and II series, but not the Plus nor (I believe) the Portable). Assuming that the Ehman case carries the terminator-power through to its second jack, the external terminator will receive power from the bus and will work correctly. There's an entirely different way to do this, which doesn't require an external terminator or extra cable... but it requires rewiring the Ehman case and/or the SyQuest interface so that the SyQuest's internal terminators can accept power from the bus if the SyQuest is powered down. This is not for the faint of heart, and I don't really recommend it... write for details if you absolutely need 'em. -- Dave Platt VOICE: (415) 493-8805 UUCP: ...!{ames,apple,uunet}!coherent!dplatt DOMAIN: dplatt@coherent.com INTERNET: coherent!dplatt@ames.arpa, ...@uunet.uu.net USNAIL: Coherent Thought Inc. 3350 West Bayshore #205 Palo Alto CA 94303