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Switchable SCSI terminator schematic

Where can I find a schematic for a terminator (SCSI-1) which can be switched on and off?

The idea is to add an external SCSI connector to my system which will either be unused (and therefore must be terminated) or connected to some external SCSI device (and must be terminated).

Instead of pluggin/unplugging an external terminator I was thinking of a more elegant solution where I have a switch to turn termination on or off.

Here's a block diagram I've made:

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Reply to
NoSp
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There are such things as repeaters which provide the termination for both directions, and these are low cost:

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The terminators are even cheaper, may be better to just bring a length of cabling out and plug a terminator into the end of that, then use an IDC tap to stub out at 10cm max for another device, the stub can be left open. Elegance has more to do with using what's readily available for the application than wistful wishing for something you're incapable of building.

Reply to
Fred Bloggs

Here is what you want

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Reply to
eyezkubed

There are ICs that implement a good SCSI terminator; just buy them, don't try to reinvent. UCC5672 or DS2119 are available at DigiKey.

Two problems are implicit in your stated requirements

(1) the SCSI-1 specification called for 220/330 ohm 5% terminator resistors, and connection to +5V, with wide error margins. Later SCSI specifications made it 110 ohm 1% terminator and connection to regulated 2.85V. This 'active' termination is what logic-switchable termination ICs implement, it's a free upgrade from the old termination.

(2) many SCSI devices have differential (either low voltage or RS-485 high voltage differential) inputs/outputs and must be disabled, using a feature of the SCSI standard called 'differential sense', if a non- differential (i.e. 'single-ended', or SE) device is attached, Your terminator IS a SE device, and should only be used when the 'differential sense' is implemented properly (i.e. you really want to adopt this part of the subsequent standards after SCSI-1).

Reply to
whit3rd

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