Wednesday, June 10, 2015

Reverse engineered the disk interface and dug further into the restoration of the drive.


I located a welding shop that I think might be able to fix the part. I will drive by there and see what they can do, but I might not get to their location today. This is plan A, repair of the bracket.

Plan B is to get a replacement part from a fellow restorer, but it has to travel from Finnland. Further, it renders one of his printers incomplete.

Plan C is to pay a prototyping machine shop to make a copy of the part, once it is scanned, measured and converted to a very precise diagram. A pivot for a secondary lever mounted to my latch is pressed on somehow, which introduces extra manufacturing steps in order to get an exact substitute.

Plan D is to drill and tap into parts on the printer in order to create a vertical tab in place of the one that broke off - this is the least aesthetically desirable of the four choices.


The four motor windings are good - no shorts or opens. However, I did notice that the pin in one of the plugs wasn't pushed into the connector all the way, thus backed out when I stuck in an ohmmeter probe. This goes to start winding number one, which if it wasn't securely connected would cause the symptoms I saw.

I have some work to do reinstalling the start capacitors and putting the power section back into the bottom of the drive, before I can test again with the reinserted pin in the motor connector. Meanwhile, I did more reverse-engineering of the interface terminator card to build out an accurate diagram for the two 50 conductor cables that form the external interface to the disk drive.

As with the RK-05 and 2310 drives, these require an external controller to generate the serial data stream that is written on the disk, and that decodes the incoming stream to read the data under the heads. The 2310 logic is embodied in the disk drive adapter in the 1130 system, while DEC schematics describe the controller used with RK-05 drives on PDP systems.

Pertec D3422 drive
These are so similar to my Pertec drive that the only variations are caused by the increased linear bit density and rotational rate. The 2310 spins at 1500 RPM but this drive operates at 2400. The 2310 implements 203 cylinders at 100 tracks per inch (cylinders per inch), but this drive is built at 200 TPI and thus has 406 cylinders. The 2310 records at 1100 bits per inch while the Pertec is packing 2200 bits per linear inch. Finally, the 2310 drive has cartridges built with eight physical sector pulses per rotation, while the Pertec drive delivers 24 physical sector pulses.

The logic for reading streams from the head is the same and the drive delivers a clocking pulse along with the data value, making it easy to retrieve the disk contents as long as the circuit can operate at the higher bit rate, 3.2 times what arrives on a 2310 and 1.6 times what arrives from an RK-05.

I can make this a compatible 2310 drive as far as the software is concerned by restricting the use of the tracks to the first 203 cylinders and by instituting a divide by three function for the sector pulses. I will be wasting a lot of the disk capacity, not only by ignoring half the cylinders, but also by storing only four sectors of 321 words per rotation whereas the Pertec can store more than double the bits per track.

My drive is described in the right-hand column, plus the 2400 rpm option
I waste more than half of the track capacity and half of all the tracks. Net, I will use less than a fourth of the cartridge capacity if I operate this in 2310 compatible mode. If I were to access the native capacity, plus make use of the fixed platter that doubles overall capacity yet again, I would have to write my own software and modify DMS to handle the device.

Continuing on the reverse engineering of the terminator board, I now have the disk to controller cable fully worked out, but it took hours to work through this and be sure it all made sense. By evening, the controller to disk cable was also documented and I felt good about my understanding of the interface overall. Nothing at all on bitsavers or the internet to document the Pertec disk interface. The manuals refer to twin 36 conductor flat cables but the terminator board has two kinds of fifty pin connector sets - one for IDC flat cable connectors and the other for wide circuit board sockets. No 36 conductor choice and the darned manual doesn't show how those work or are connected.

I spent the early evening cleaning and checking out the power circuitry, securing the two motor start capacitors in place and inspecting all the wiring and parts. The goal is to complete reassembly of the lower power section and then test again to see whether the motor is starting or I have to dig into the motor control and other logic.

At the end of the evening, all is reassembled and the motor is not starting. Time to figure out some diagnostic signals to watch in order to track down the reason that startup does not occur. I got to the point where I had to monitor relay K1 - the emergency retract relay - to be sure it was engaging. It didn't seem to pick on the first try, flickered at the end of the start cycle, then when I tried again I saw a puff of smoke and a burned component smell from roughly the area where the relay sits.

Good news and bad news. This will certainly help me narrow in on the component(s) that are bad, but I have to replace the parts whose funeral pyre produced the white smoke.

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