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Using both active and passive termination on same SCSI bus

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First post, by auron

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SE SCSI supports different speed devices on the same bus without speed drop. this is clear from page 3 of this paper: https://www.microsemi.com/document-portal/doc … csi-white-paper

as far as termination goes, it's generally stated that passive termination is no longer permitted/recommended starting with SCSI-3, but i can't find any clear info on whether that means that putting a single SCSI-3 device on a bus will put that requirement in place for both ends of the bus or not. as an example, take a SCSI bus with just two internal devices, a 50-pin SCSI-2 CD-ROM with external resistor packs on it and a SCSI-3 UW 68-pin drive with internal termination. the controller will be set for high byte termination in this case of course.

i've ran this exact setup for a while in the past, with a short 2-connector 50-pin cable as well, and had zero issues with it, but i'm wondering if people would still go out of their way to remove those resistor packs and put external termination in place. now putting a bunch of ultra devices before said SCSI-2 drive and still terminating with those resistor packs would probably be much more problematic.

Reply 1 of 1, by mkarcher

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Generally, it is advisable to use active termination on both ends of any bus with at least one device that is SCSI-3 capable. The idea of the terminators is to prevent the reflection of signal edges at the end of the cable. SCSI-3 capable devices make sharper signals, and reflections of these sharp signals at classic passive terminators is not guaranteed to be sufficient.

In your scenario, the UW hard drive could create signals that get partially reflected at the passive terminator in the CD-ROM drive, so the controller will get an overlay of the direct hard drive signals and some interference from the reflections at the CD-ROM drive. This might cause signal integrity issues. I expect here is a good chance that you won't have any problems with that setup, but I guess no one is going to take the risk to "advise" or "recommend" a setup like that.

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