EPS 16-Plus compatibility with the Iomega ZipDrives

Ensoniq has posted on their compatibility list, from time to time, that the Zip series from Iomega is NOT compatible with the EPS 16-Plus. This is true if you simply hook a ZipDrive alone with a 16-Plus equipped with their Ensoniq SP-1/2 SCSI Interface. However, the only problem is that the data connection is not strong enough for reliable SCSI transmission. A ZipDrive CAN work with an Ensoniq EPS 16-Plus!

However, there is hope. Don't let the company tell you what to do! (Especially when your keyboard is out of warranty, and the company doesn't exist any more...) There are 4 possible solutions to this dilemma:

SCSI Interface Term Power Modification (Chris Gandy)
"Connected a schottky diode (a 16A power one, coz that's all I had) from the +5v supply line on the SP-2 board to the appropriate pin of the 26 way connector on that board. I used a Schottky diode because of the low voltage drop. The termpower line on the SP-2 board goes straight into a capacitor on the top of the board, as does the +5v line, so I wired the diode between the appropriate legs of each of these capacitors - which happen to be separated by about half an inch. The only reason for using a diode at all is to prevent some other peripheral, which does supply termpower, from potentially becoming the +5v power supply for my EPS16+! (which would probably exceed its current capability). The diode only lets current flow in one direction, from the EPS's +5v power supply _into_ the termpower line, but it will not let current from an external source flow back into the EPS's power supply, which, of course, feeds much of the other circuitry in the EPS.

Concerning the Schottky diode; there are several types of diodes. The most common, nowadays, is the silicon diode (in its various forms), and when current flows though this, a voltage drop of about 0.7 volt (or more) appears across the diode - a true 5v supply would only put 4.3v (or less) on the termpower line, which would not be ideal since it is specified to have +5v (I don't recall the specified tolerance, but it's probably smaller than +/- 0.7v). There are some other types of diode which perform the same function but give a considerably smaller voltage drop, and the Schottky diode is one of them. This uses a metal/semiconductor junction rather than the p-semiconductor/n-semiconductor junction of the normal silicon diode, and achieves a voltage drop of about 0.2v (or less). In my case, I will probably never connect another peripheral to the SCSI port, so I could have just used a wire, but it seemed like a reasonable safety precaution (I have read that others have put a low-current fuse in series with the diode in applications like this, which is another reasonable safety precaution - but I didn't bother).

My Zip drive wasn't willing to format a ZipDisk until I applied term power. Before I committed myself to the modification with the diode, I really did apply external termpower, using a pair of 50-way Centronics (i.e. SCSI I) connectors connected back-to-back by a short 50-way ribbon cable. With this arrangement, I was able to tap into the termpower line (pin 26 on the 50-way) and one of the many ground lines (I needed to rig this up for another problem I was trying to solve, which is the use of the ICD 'Link II' SCSI interface for the Atari ST computer with a Zip drive - similar problem; lack of termpower from the drive). I found that applying +5v to the termpower line sorted out the problem with formatting, and since then I haven't looked back! I don't know if the EPS is happy to save and load files to the Zip without the termpower - but I guess I could find out if that was important."

Specifics (Alan Brown)
Firstly, locate the capacitor marked C2 on the SCSI board (SP-2). It should be a 0.1uF. The left leg of this should be +5v. (Check with a multimeter if you can - otherwise use a simple continuity tester to verify that this point connects to Pin 5 of the power supply connector on the power supply board to the left of the SCSI board).

Next locate C4, another 0.1uF cap. The right leg of this should be Termpower. Again double-check with a tester to see that this does indeed go through to Pin 25 of the SCSI port (Bottom left pin looking at the EPS-16+ SCSI port). The reason for this double-checking is that if you have a 3rd party SCSI, the component labeling may be different - I have a PS systems one and the caps are C1 & C2!

Just connect the diode (with some extra wire & insulation) from C2 to C4 (banded end facing C4). I chose these caps as they were the most accessible for soldering. In fact you could even solder from the power supply connector itself.

If you are ONLY ever going to use a Zip drive, a wire will suffice. The last thing to remember is, don't try to format a Zip tools disk, unless you long-format this on a computer first. All other off-the-shelf disks should format fine.

If you have any questions, please check with RCS.

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