Limit on daisy-chaining parallel port devices? \ VOGONS

Limit on daisy-chaining parallel port devices?

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First post, by snorg

Posted on 2018-09-16, 17:48

snorg Offline

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Lots of parallel port devices/drives have a pass-through port so that you can use the drive with another parallel port device.
Which makes me wonder, is there a limit on how many you can daisy-chain this way? I know with SCSI you could do seven devices but SCSI was designed for that, the parallel port (as far as I know) was only ever meant for printers and was adapted for other uses. So I imagine you could do at least 2 but I would be curious what the limit is. Anyone out there have several LPT devices hooked up like this?

Reply 1 of 4, by derSammler

Posted on 2018-09-16, 18:08

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There is no limit, since daisy-chaining was never intended. The port is designed for a single device. Daisy-chaining works until the voltage drops below the point where the other side can no longer distinguish between TTL high and low.

Reply 2 of 4, by snorg

Posted on 2018-09-16, 18:51

snorg Offline

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derSammler wrote:
There is no limit, since daisy-chaining was never intended. The port is designed for a single device. Daisy-chaining works until the voltage drops below the point where the other side can no longer distinguish between TTL high and low.

Ok, so there is no set limit as such, but would it be fair to say a practical limit is maybe 2 or 3 devices?

Reply 3 of 4, by ibmapc

Posted on 2018-09-16, 19:06

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I think you would be limited by the types of devices chained together as well. You would not be able to use more than one of a particular device type. I.E. you won't be able to chain two Parallel Zip drives or anything that uses the same resources.

Greg.

Reply 4 of 4, by Zup

Posted on 2018-09-16, 19:12

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I think that the limit is TWO devices, and one of them must be a printer.

I've only seen passthrough ports in external drives and scanners, but I don't think that you can chain two of those bidirectional devices.

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