Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uwm.edu!psuvax1!rutgers!umn-d-ub!cs.umn.edu!chou From: chou@umn-cs.cs.umn.edu (Chih-Hsiang Chou) Newsgroups: comp.sys.mac.hardware Subject: Re: Can the SCSI bus be terminated w/o powering up noisy device? Message-ID: <1990Feb16.202333.28586@umn-cs.cs.umn.edu> Date: 16 Feb 90 20:23:33 GMT References: <17008@boulder.Colorado.EDU> Organization: University of Minnesota, Minneapolis - CSCI Dept. Lines: 150 In article <17008@boulder.Colorado.EDU> fozzard@boulder.Colorado.EDU (Richard Fozzard) writes: >A question for you SCSI jocks out there: > >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. You're right. The terminating resistors, being connected to the Ehman's power supply, ground the SCSI bus if not powered up. > >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. The fix is to disconnect the terminating resistors from the Ehman's power supply and feed them power from the SCSI bus (pin 25). I had done this successfully, with authorization from Ehman's technical support, and have no problem so far (near a year). Whenever I need to use my Syquest drive, I simply power-up the drive, insert the cartridge, wait the motor to speed up, then use the SCSITools DA to scan the bus and mount the cartridge volume. No reboot needed. > >If you really understand SCSI termination, a reply to the net would >probably be widely appreciated, since to most of us SCSI termination >is black magic. > I have enclosed a copy of the procedures of my modification below for anyone who is interested in doing so. The modification is recoverable so you won't void your warranty. However, there are three things to note: (1) the drive I have is the early Ehman model in PC style box, but I doubt there is any significant change in the new model. (2) be VERY carefully about the power from the SCSI bus. The first time I did, I forgot to disconnect the terminating resistors from the Ehman's power supply, and I blew away a 1A soldered fuse and one printed circuit wire on my Mac II motherboard. :-( The repair will cost serveral hundred $$$ for a motherboard replacement, but I fixed it by myself anyway (less than $1 for parts). (3) do this at your own risk, no guarantee or responsibility is promised here. However, if you have a success, I do like to hear from you. --- Instructions for Terminating Resistors Change on Syquest Drive ------ 1. Disconnect the power cord and SCSI cable from the Ehman case. 2. Unscrew the 6 screws on the bottom of the Ehman case and slide the cover out. Mine is the old model (PC shape), your packaging may be different. 3. Take a detail top-view look of the internal construction of the Ehman case with the opening of the Syquest drive facing to you. The Syquest drive is located at the lower right corner of the Ehman case. The power supply is at the left side of the case which has wires connected to the Syquest drive. There is also a ribbon cable which connects between the 25-pin SCSI port on the Ehman case and the 50-pin SCSI port on the Syquest drive. You may want to disconnect them temporarily to allow more working space. 4. The terminating resistors (two of them) are located at the back of the Syquest drive (left side, if you keep the top-view). Each of them is an array of resistors in a comb-like packaging. They are socketed in parallel. You should be able to remove the resistor one by one by gently shaking it while pulling it out. The +5V DC power is connected to the leftmost (again top-view) pin of each of the resistors. You may want to validate its existence by a voltmeter. 5. Now, if all is possible, find two new sockets which are identical to those on the drive. Insert the resistors into the new sockets with the leftmost pin open. I.e. first align the resistors with the new sockets, shift the sockets one pin to the right, then insert the resistors. See the diagram below: ________________ |______________| Terminating Resistor | | | | | | to pin 25 ___| of Ehman SCSI ________________ port on case |______________| New Socket ________________ |______________| Original Socket +5V from ____| Ehman power supply Next, insert the new sockets into the original sockets with the open leg on top of its original position. In the final view the new sockets are sandwiched between the terminating resistors and the original sockets. Be sure that the open legs do not touch the original sockets. Since I can't find the identical sockets, I home made one from another type of connector. I forgot the name. It's a long array of connector just looks like the terminating resistor. You may omit the new socket if you can bend the leftmost pin and don't have it touched with the original socket when you insert the resistor back. But for easy recovery to its original connection and shape, I chose to use the new sockets. 6. The +5V DC power from the SCSI bus is on the 25th pin of the Macintosh SCSI connector. Note that old models of Mac don't have this power line available. Since Ehman uses the same 25- pin connector, it's rather easy to find it. Simply connect a wire between the pin 25 of the SCSI port (a good place is on the small printed circuit board which connects the two 25-pin SCSI ports on the Ehman case) and the open legs of the terminating resistors. I wire-wrapped the connections instead of soldering it, for reason of easy recovery (in case I need warranty service). You may want to connect the wire first before insert the resistors back to the sockets. 7. Validate all the connections, then reconnect all the internal wires (if they were disconnected) and power cord. Don't connect the SCSI cable from the Mac to the Ehman case yet. 8. Turn on the Ehman's power supply (no cartridge inserted). Make sure there is no +5V coming up at the open legs. 9. Turn off the Ehman, connect the SCSI cable, then turn on the Mac. Make sure +5V is on the 25th pin and the open legs. Also make sure other SCSI devices such as the internal hard disk can boot up when the Ehman is off. 10. Turn on the Ehman, make sure everything is still OK. Now you can insert a cartridge and reboot the system (or use SCSITools to mount the cartridge). Make sure everything is doing fine. 11. If you have success at this point, congratulations. Otherwise, don't blame me. 12. Power off all machines. Disconnect all external cables if necessary. Put back the cover, screw it, reconnect all the cables, then your're done. That's it. GOOD LUCK!! P.S. I assume your Ehman drive is at one end of the SCSI chain, otherwise the terminating resistors should be removed completely. -- Chih-Hsiang Chou chou@umn-cs.cs.umn.edu Department of Computer Science University of Minnesota