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Will this SCSI terminator work plugged into the logic board?
Posted by: MrFahrenheit on 2021-12-09 20:05:09
I’m really looking for a solution that allows me to plug just a terminator into the logic board without any cable. I found this:

www.ebay.com

Internal SCSI Active Terminator IDC 50-pin Male to Female Pass-Thru | eBay

New, never used.
www.ebay.com

It says it’s pass through and has male and female ends. Does that mean it terminates regardless of which end is plugged in? I want to use the female end directly into the SCSI connector on the logic board of some Macs to terminate internal SCSI. Specifically I have found that they G3 beige logic board acts up when an external SCSI device is used without a terminator on the internal bus.

Thoughts please.

Alternatively I’d ask someone to make a PCB design and upload to GitHub, like this one, to be made by hand:

28EF9D47-9A71-4F3F-BEA9-0870CF5AF5CB.jpeg985E44C3-39FB-441A-B79C-87A511A8A9D7.jpeg255BAAF8-B6E8-40B2-A786-1F3F2F57598A.jpeg
Posted by: trag on 2021-12-09 21:54:43
It shouldn't matter which end is plugged in, it's providing a connection of a certain resistance between the bus signals and a fixed voltage at that point in the bus.
Posted by: MrFahrenheit on 2021-12-10 04:43:07
It shouldn't matter which end is plugged in, it's providing a connection of a certain resistance between the bus signals and a fixed voltage at that point in the bus.

That’s what I was thinking, too. Thanks!
Posted by: Trash80toHP_Mini on 2021-12-10 08:25:07
Interesting problem, do you have any internal devices on the SCSI bus when it gets wonky? Apple provided a single ended gray terminator for many Macs in the case that there would not be an internal device terminating that end of the bus.
Posted by: Stephen on 2021-12-17 09:45:22
Here's a PCB
Alternatively I’d ask someone to make a PCB design and upload to GitHub, like this one, to be made by hand:
Here's a PCB design.
Posted by: ymk on 2021-12-17 09:54:21
Is your external device terminated?
Posted by: MrFahrenheit on 2021-12-18 10:41:31
Interesting problem, do you have any internal devices on the SCSI bus when it gets wonky? Apple provided a single ended gray terminator for many Macs in the case that there would not be an internal device terminating that end of the bus.

No, nothing connected internally. Not even a ribbon cable.
Posted by: MrFahrenheit on 2021-12-18 10:42:02
Here's a PCB

Here's a PCB design.
That's pretty awesome! Thank you!

One question, could the resistors be mounted on the opposite side of the PCB to the connector?
Posted by: MrFahrenheit on 2021-12-18 10:43:06
Is your external device terminated?

Yes. Using good quality active terminators, as well as traditional Apple branded grey and black ones. Same experience. Only is bad on some Macs and is usable on others without any internal termination. But at least on a beige G3 logic board, I need to add internal termination if no device is present, for external drives to boot.
Posted by: Stephen on 2021-12-18 12:29:21
That's pretty awesome! Thank you!

One question, could the resistors be mounted on the opposite side of the PCB to the connector?
Yea, should only be a matter of soldering them on the “bottom” (opposite side” in the correct orientation.
Posted by: Trash80toHP_Mini on 2021-12-18 16:19:51
Very nice PCB, that'll work like a charm! Macs without internal SCSI devices shipped with the gray terminator insert installed.
Posted by: Fizzbinn on 2022-01-03 08:40:40
I'm looking for an internal terminator for an original Duo Dock. Sometime in its past it looks to have had an internal hard drive and a Nubus card installed which are no longer there, terminator likely got tossed when the HD was added. This is the one from my Duo Dock II:

3BA592B6-0D3A-4708-B8EA-595A35D2F89A.jpeg

Question though, is the green "820-0408-01" apple internal terminator an "active" terminator?

At least from Apple's Duo Dock II dev note this seems to be required:

"If an internal hard disk drive is not installed, an active terminator plug is inserted in the SCSI connector to provide termination for the Duo Dock II side of the SCSI bus."

Active terminators have a voltage regulator not just resister packs, correct?

In another thread @MacKilRoy pointed me to:

www.ebay.com

Active 50pin/wire Internal Male~Female cable/cord Terminator SCSI1/2 HD/CD Drive | eBay

SCSI Internal 50-pin (aka IDC50) Male to Female. Need an Internal Cable with this connector?. Need other Internal SCSI Adapters?. Need Internal SCSI Cables?. Picture shows 2, you get 1.
www.ebay.com

Which does say "ACTIVE" on the side.
Posted by: Trash80toHP_Mini on 2022-01-05 08:03:52
Not sure about active termination implementation, that's be @max1zzz territory. IIRC he did active termination on a project?

Searching this thread for you I found another one from 5 years ago on the related Black IIfx Terminator front. All the Docs seem to be there. I was wondering if adding a passthru to the internal PCB might be encased in a printed block to fulfill IIfx external termination requirements?

Anyone able to make an Apple SCSI Terminator II (Black) - M5871G/A?

For the benefit of those with Mac IIfx's (or NTX printers) out there that having issues getting external SCSI devices to work because they can't locate the incredibly rare "Black" Apple SCSI Terminators II's - I was wondering if the linked Apple Tech notes might be of interest to someone here...
68kmla.org 68kmla.org
Posted by: max1zzz on 2022-01-05 08:50:44
Not sure about active termination implementation, that's be @max1zzz territory
Active termination is easy you just pull each line on the scsi bus up to +2.85v through a 110 ohm resistor
Active terminators have a voltage regulator not just resister packs, correct?
Yep active terminators will have a little vreg on them to generate the +2.85v
Posted by: Trash80toHP_Mini on 2022-01-05 08:53:17
THX. Can you do a PCB for the IIfx that's also active?
Posted by: max1zzz on 2022-01-05 09:32:49
THX. Can you do a PCB for the IIfx that's also active?
Yeah that wouldn't take long, the the IIfx one passthrough or not?
Posted by: Fizzbinn on 2022-01-05 09:44:53
Yeah that wouldn't take long, the the IIfx one passthrough or not?

So are there three types of internal terminators we are talking about:

Passive: The "820-0408-01" clone @Stephen made (PCB design) - I'm assuming this one is passive as it doesn't have a voltage regulator or 110 ohm resistor packs.

Active: A potential clone of the gray Data Mate that Apple put in Duo Docks and potentially other System with no internal HD, which could be a modification of @Stephen's design with a voltage regulator and 110 ohm resistor packs I suppose.

IIfx Active: which I'm not sure I'm following on what is different/needed there?
Posted by: olePigeon on 2022-01-05 11:32:33
IIfx Active: which I'm not sure I'm following on what is different/needed there?
I was referred to this thread at VCF. Apparently, "the only difference is the black terminators had a bypass cap on the 5v line." Take it as you will.
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