Date: Sun, 28 Mar 1999 22:10:08 -0500 From: "Christopher R. Bowman" <crb@ChrisBowman.com> To: Darren Reed <avalon@coombs.anu.edu.au> Cc: hackers@FreeBSD.ORG Subject: Re: another ufs panic.. Message-ID: <199903290319.WAA13485@quark.ChrisBowman.com> In-Reply-To: <199903282241.IAA00135@cheops.anu.edu.au> References: <37231.922658651@zippy.cdrom.com>
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At 08:41 AM 3/29/99 +1000, Darren Reed wrote: >In some mail from Jordan K. Hubbard, sie said: >> >> > If this is an aha2940 or similar controller, then it doesn't support using >> > all three connectors at the same time. Using both internal connectors at >> > the same time as the external one is just asking for trouble. >> >> Let me just echo this claim. Back when I was somewhat younger and >> less experienced in the ways of SCSI, I tried to do this for the >> simple reason that it's the obvious thing to try when you have both >> wide and narrow peripherals in the box (like a wide drive and narrow >> cdrom), an external device like a scanner and absolutely no conception >> of how the internal busses are actually wired. What happens then is >> that you create a "Y" in your SCSI chain, with two terminators on one >> end, and the fact that it worked for me at all for 5 months until I >> got another drive for the wide chain and totally pushed things past >> their limits is, frankly, pure amazing luck. > >Sigh :-( It was all working so lovely too...which did you get a new one >for, narrow or wide controller ? Darren, while it is true that you can only use 2 of the 3 connectors on your card, you ought to be able to mix wide and narrow devices on the internal and external cables. However, you must, as always, be sure that both ends of your chain are terminated properly. Since, electrically, all the connectors are on the same bus, if you have a wide drive on either of the 2 connectors that you choose to use, you must have wide terminators at both ends of the bus. Lets assume that you have a wide device somewhere on your bus and for the purpose of this discussion when I say a device with it's terminators turned on I really mean a device that has built in terminators and has them turned on (usually done by adding or removing the resistor pack or moving a jumper) or has 2 connectors on it, and has a terminator plugged into one of the connectors. Then there are basically 4 case: 1) you have a wide cable that has a terminator physically built into one end of the cable. Into this cable you can plug wide and narrow devices(narrow ones will require a little plastic wide to narrow connector converter) none of which may have their on board terminator turned on, because, of course, the cable provides the termination. (NOTE: the end of the cable farthest from the terminator must be plugged into the SCSI card) I have only seen this on internal ribbon connectors, but this is how my Toshiba Equium 6200M came and I have a narrow CDROM and 2 wide drives on the cable. 2) you have a wide cable that does not have a terminator physically built into the cable. Again you can mix wide and narrow devices on this cable as above with their terminators turned off, but in this case you should either plug a wide terminator into the last connector on the cable or place a wide drive with its terminator on into the last connector on the cable. This will terminate both the upper and lower byte of this end of the bus. If you have a narrow terminator or a narrow device with it's terminators on in this last connector you will not be terminating both the upper and lower byte of the bus. 3) you have a narrow cable (which of course can only have narrow devices on it). Again the last connector must either contain a terminator or a device with it terminators turned on. But since we assume a wide device somewhere on the bus (it can't be on this cable so it is on the other one) we must terminate the upper byte since the termination at the end of the narrow cable only terminates the lower byte. In this case we must also turn on the SCSI card's own on board terminator for the upper half of the bus on the connector using the narrow cable. This is done either via a jumper on the board, or via a bios option. 4) either the external or BOTH internal connectors are not being used in which case since we again are assuming a wide device on the bus, we must turn on the upper and lower terminators on board the SCSI card. If the one cable is an internal connector we turn on both the upper and lower external terminators and vice versa if instead the external connector is in use. as a special case if no wide device are in use externally or internally then you need not terminate the high byte of the bus on any connector. If you just remember that the SCSI bus cannot be a Y (thus only 2 out of 3 connectors) and that the SCSI card actually, electrically, sits in the middle of the bus and not the end, even when only one connector is in use, and that both ends must be terminated, you should come up with the right way to set up the termination. -------- Christopher R. Bowman crb@ChrisBowman.com http://www.ChrisBowman.com/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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