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I previously asked this question in the BlueSCSI thread but it's a bit off-topic, so I'm continuing here instead. I have two external SCSI devices that I want to use at the same time, the BlueSCSI and an external Ethernet card. But only one SCSI port to connect them to. @erichelgeson suggested using and old SCSI enclosure, and I've looked around, but they are quite pricy. And big.

 

So I'm trying to understand how the SCSI chain works and see if I can make my own "hub". Are all devices hooked up in parallel? Can I just take a breadboard and wire up two female DB25 to one male DB25? And then put termination on the last SCSI device in the chain? What if one of the devices has its own power source, do I also need to add diodes?

Edited by blindowl
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While there are such things as SCSI hubs, they're not what you think and are meant for really old fashioned shared media setups, like from the first half of the 1990s. The amount of extra equipment is huge, and it's definitely not what you want.

 

That said, you just need a way to daisy-chain the devices. If the external SCSI ethernet device has a 50 pin ribbon connector, you just need a cable to go from 25 pin D to two 50 pin ribbon connectors. An external enclosure that'll take both devices is a good idea, but if pricey, you can find just a SCSI cable on eBay for cheap.

 

You can also get 50 pin SCSI external connector (HPDB50 or CN50) to two 50 pin ribbon connectors (IDC50), then get a 25 to 50 pin external SCSI cable. This might make it easier to find. For example:

 

https://www.ebay.com/itm/CablesOnline-3ft-DB25-25-Pin-Male-to-CN50-50-Pin-Male-SCSI-Cable/270840782925

 

https://www.ebay.com/itm/50-Pin-SCSI-Flat-Ribbon-Cable-Up-to-7-Internal-Drives-2-External-Connector-7ft/164676604827

 

Just examples. For instance, you can probably find the equivalent of the second link with just two 50 pin ribbon connectors, which I'm guessing would be fine for you since at least the BlueSCSI has its own termination.

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Yeah, SCSI devices are just wired in parallel, so you can do what you're suggesting with a breadboard. All the SCSI cases with two external ports I've ever taken apart have just been a single ribbon cable with external connectors crimped to either end and a 50-pin IDC connector in between. You might even be able to get the parts to make that yourself.

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I have a tape drive enclosure with the wrong SCSI connectors but OK power supply. I have a 50-pin SCSI CDROM drive. I have some Centronics IDC female connectors (C50F).

 

I'm thinking of taking the tape drive out of the enclosure and using just the power supply. Then if I take a 50-way IDC (2 rows of 25) cable such as found inside a Mac SE/30 and crimp a C50F onto it, can I just connect the drive via this ribbon cable connected to a standard Mac SCSI cable 25-50 SCSI cable? If a drive has a termination jumper does that mean it can be connected as the only device, no need to worry about a terminator on the cable?

 

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On 4/14/2021 at 4:29 AM, ymk said:

SCSI devices are wired in parallel, but the physical layout matters.

 

Wiring devices in a star or T layout is asking for problems.

What kind of problems? What's the best way to wire the devices?

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20 hours ago, blindowl said:

What kind of problems? What's the best way to wire the devices?

 

Signal reflections.  The devices should be laid out in a line:

 

End of 50-pin cable -------- middle of 50-pin cable -------- SCSI controller on logic board -------- DB25 external connector ------- external device 1 ------- external device 2...

 

Branches (as in a USB hub) should be avoided.

 

Adapt the DB25 to a 50-pin header, like the logic board uses.  Then you can connect both external devices using a 3-connector 50-conductor cable and terminate the one on the end.

Edited by ymk
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