Make your own Macintosh IIfx Black SCSI Terminator 590-0705-A

[David Cook]

The Macintosh IIfx has a special black SCSI terminator compared to the standard passive terminators. Unfortunately, the black terminators list for upwards of $200 on eBay. Furthermore, there is confusion and myths surrounding whether they are necessary and how they actually differ electronically from standard passive terminators.

Fortunately, Apple provided the exact circuitry and differences in Macintosh "Technical Note DV 15 - SCSI Termination" (attached).

There are three differences:
1. The addition of a 2.2 uF capacitor connected between terminator power and ground
2. The addition of a 0.01 uF capacitor connected between terminator power and ground
3. The reduction in pullup resistance only on the REQ pin, dropping from 220 ohms to 150 ohms. Note the other pins remain at 220 ohms.

The purpose of these changes is to prevent the REQ pin from accidentally dropping low when a bunch of other SCSI pins are quickly pulled down intentionally, as might happen at high speed. The capacitors provide a little bit of local power storage to supply during rapid changes, and the lower resistance on the REQ pullup makes it a little harder to pull down.

Many people have already pointed out that an active terminator will do an even better job, as it has even lower resistance pullup resistors, a voltage regulator, and capacitors. Now that you know what changes are in the IIfx terminator, you can be confident that the advice to use an active terminator is correct.

However, maybe SCSI voodoo has a grip on you, and you're sure that a black IIfx terminator would fix the issue. Well friend, you can make your own by modifying a successor passive terminator released by Apple. Buy a 590-0695-A on eBay for $12 plus shipping.

Cut open the terminator casing using a Dremel with cutoff wheel or your choice of tools. For example, slit the side of the gray rubber to pop it off. Then cut the metal shielding foil and peel it back. Undo the tape that holds the tan plastic inserts.



The first surprise is that this Apple passive terminator already includes the correct capacitors! So, this is an upgrade from the standard passive terminator.



Flip it over to notice the interesting resistor packs have dual 220 ohm / 330 ohm resistors. I guess there was enough demand for SCSI terminators to manufacture special parts like this.



Flip the terminator back to the side with the capacitors. All you need to do is solder a 470 ohm resistor between the center pin (TERM power) and pin 48 (REQ). A little heatshrink tubing or kapton tape prevents a short.



A 470 ohm resistor in parallel with a 220 ohm resistor (already installed) = ~150 ohms.

Reassemble the terminator. I had to nibble a bit of the tan plastic insert away to compensate for the space taken up by the resistor and heatshrink tubing. For the metal wrapping, you can resolder the peeled shielding or get a roll of grounding tape (aka EMI shielding tape, aka double-sided conductive copper foil tape) from Amazon.

Or, maybe just buy an active terminator like Apple 590-0772.

If this has helped you, all I ask is that you ping me if you run across any 16 MB IIfx memory SIMMs for sale. My poor IIfx currently sits at 20 MB. : )

- David

 

[jmacz]

@David Cook thanks for this. I opted for an apple active terminator.

Question though, why are the black ones so expensive, when the active ones are much cheaper? Is it just a collector’s item?

Oh, another question. What about internal termination? If I have multiple internal devices, say a scsi rotating drive, internal scsi cd rom drive, internal scsi magnetic optical drive, or even a Zulu scsi, would their termination jumpers work or have a similar issue to these passive terminators?

 

[jmacz]

Tagging others who might know. @MrFahrenheit @joshc @Phipli @bigmessowires

 

[Phipli]

I've never found a primary source on the topic.

I think some apple pass-through grey terminators are active, some are passive (I'm likely to be wrong).

It would be nice to have a definitive answer, but "use an active terminator" is probably reasonable.

 

[cheesestraws]

Question though, why are the black ones so expensive, when the active ones are much cheaper? Is it just a collector’s item?

Yeah pretty much, there's a mystique associated with them from being the terminator that goes with the IIfx.

 

[MrFahrenheit]

They’re expensive likely because
A) People think they’re necessary but don’t know active is just as good/better
B) Are more scarce resulting in higher demand
C) Have been priced this way due to eBay sellers like Mr Polkadots and so the market is trending with his sales

No need for a black one, a proper active terminator is just as good. The granite digital ones are really high quality and cost less.

 

[jmacz]

Thanks.

Anyone have an answer on internal termination?

 

[David Cook]

Thanks.

Anyone have an answer on internal termination?

It varies from drive to drive. Most of the early drives were simply passive resistor arrays. Newer drives had an active terminator integrated into the SCSI interface chip.

 

[David Cook]

I purchased a IIfx black terminator from @joshc. He carefully packaged it in multiple layers of padding so that it would safely arrive from overseas. He's not going to like what I did to it.

Here it is, an intact official Apple IIfx black terminator 590-0705-A.



I cut the rubber grip at the seam line using a scalpel. I then peeled back the metal shielding using pliers. Underneath is a plastic shell held together with masking tape.



As expected, here we see the capacitors mentioned in Apple's Technical Note. They look just like those on the gray Apple passive terminator 590-0695-A.



On the other side, we see the resistor arrays. They also look just like those on the gray Apple passive terminator. Hey!!! Wait a minute. I thought this was supposed to be a fancy IIfx terminator? Where's the special 150 ohm resistor for /REQ? This is just an ordinary gray Apple passive terminator. Those cheats!!



Let's look more closely. Yup. The classic 220 ohm / 330 ohm resistor array. Standard SCSI terminator.



But, if you fold back the other resistor array, we can see it is marked slightly differently. It includes an extra 470 ohm resistor to pull up /REQ a little bit more. In fact, that's exactly the mod I made to the gray Apple terminator at the beginning of this thread. So, yes, the black terminator is different than the gray terminator. Indeed, the black terminator matches the Apple technical note.



@joshc will be happy to hear that I then reinstalled the plastic covers, applied copper shielding tape (very nice), and placed the black rubber grip back on.



There we have it. The IIfx black terminator matches the technical note but doesn't contain anything magical or more special. A gray terminator can be easily modified to provide the same performance. Not worth paying extra except as a collector's item.

An active terminator provides better performance, even on a IIfx.
1. It also has capacitors
2. It uses less power
3. It has cleaner current sources on all pins. (Regulated 2.85V through 110 ohm resistors vs 220/330 voltage divider from an unregulated 5V source)

- David

 

[jmacz]

I purchased a IIfx black terminator from @joshc. He carefully packaged it in multiple layers of padding so that it would safely arrive from overseas. He's not going to like what I did to it.

LOL

It varies from drive to drive. Most of the early drives were simply passive resistor arrays. Newer drives had an active terminator integrated into the SCSI interface chip.

The notes you mentioned about why the black terminator is different seem to apply to the internal SCSI as well as the external. So whether I have a single HD inside, or multiple HDs inside, or in my case, an HD (ZuluSCSI) and a Fujitsu Magneto Optical Drive on the internal SCSI chain, I would think I also need them to have termination that prevents the REQ pin from accidentally dropping low. What's not clear to me is what the ZuluSCSI does for its optional onboard termination (jumper) or what the MO drive does. Or do I need to disable both of them and stick my own terminator (like the IIfx one) at the end of the internal chain? Curious what Apple recommended be done with the internal SCSI chain when they released that technical note.

Even for external, what does the termination switch like the one found on an external Zip drive do? Would that be equivalent to the passive normal gray terminator in which case it's better to not use the built in Zip drive one and use an active or a passive with this modification?

 

[joshc]

He carefully packaged it in multiple layers of padding so that it would safely arrive from overseas.

Woo!

He's not going to like what I did to it.

Oh…

@joshc will be happy to hear that I then reinstalled the plastic covers, applied copper shielding tape (very nice), and placed the black rubber grip back on.

Phew. What a rollercoaster.

As usual another great write-up, you’re really creating the ultimate source of info on SCSI terminators.