Barely back to the shop after extended time away, week ending April 20, 2014


I was away until late on April 16th on a long awaited vacation. Having brought only tablet and portable computers, the progress I could make on the project was limited - no electronics assembly or test gear, no 1130 system, no peripherals nor any arduino or fpga boards. I did create almost all the logic for the fpga side of the disk drive emulator.


While on vacation I built all the logic for the fpga side of the disk drive, both functionality and timing controls that will cause this to operate at precisely the speed of the original physical disk drive in the 1130. My logic models the spinning disk and timing of words, sectors and mechanical arm movements (seek), pausing various operations until the moment when each would have occurred on the ramkit (2310) drive.

The simulator was a good tool to check out the various state machines and other logic for parts of the disk adapter/emulator. It could be run while I was away on vacation, before I returned and had access to the real hardware.

Once I was back, I looked over the code in the Arduino with its SD card and ramboard extensions, to validate that the behavior in the Arduino software matched the hardware logic I build into the fpga. I also needed to finish soldering together the ramboard which gave me 1.5MB of SPI link addressable RAM to hold a virtual disk cartridge during its use on the drive. The SD card hardware stores 100 of the virtual cartridges, with my Arduino code allowing me to mount one cartridge at a time onto the emulated drive.

The RAM board building went smoothly and soon I had the stack of Arduino, ramboard shield and SD card shield assembled, ready to do further testing. My startup code in the Arduino exercises the ramboard and seems to work properly while configuring all eight chips.

Three layers of SD card, Arduino and ram board

Assembled stack ready to operate
The main protocol between the fpga and Arduino were next to test. I had a bit of tweaking to do to the code in the Arduino to match what I instantiated in the fpga and to add the new 'unmount' transaction where the Arduino can tell the 1130 that a disk drive has been switched off and the virtual cartridge unmounted. I found that I was making decisions or setting signals too late to be swapped from the fpga to the Arduino during each SPI transaction.


The mechanisms I built into the quick disk adapter that interface with the 1130 to handle the XIO commands, read/write to memory, cycle stealing and interrupts was built with the aim that, after debugging, it could be dropped intact into the SAC adapter that interfaces with 1403 printer adapter logic. The 1403 uses cycle stealing to fetch print lines, much as the disk drive uses cycle steal to read or write words to disk, thus the requirements were very similar.

I will wait until I have shaken out the code with the disk drive before continuing my debugging of the printer interface.


Technikum29 Museum 1130

I did some investigating of the possible schematic for a missing SLT card that is needed to restore an 1130 system sitting in a museum in Germany. I made use of circuit simulators to build the proposed schematic and variations, projecting the behavior of each alternative. It appears possible to complete the design via this method, allowing the museum owner to build a substitute card.

After I selected reasonable transistor substitutions for the IBM versions - which had only the internal IBM numbering on them of type 139 and type 194 - and simulated their operation, I ordered a small supply of these to be delivered in my absence. I have these in hand now and will prototype the circuit to validate proper behavior. Once proven out, I can ship the parts to the museum for their use.

PDP/8 replica from SBC6120 and FP6120 kits

I picked up a kit to create a replica PDP/8 system - the SBC6120 and FP6120 created by Spare Time Gizmos, when an owner of the kit, determined he was not going to take on the project and offered to sell it to others. I contacted him, completed the deal and have it in hand.

The kit was not created as a full set of parts, requiring the builder to locate many components before construction. The forum for builders of the kit had good references to major distributors like Digikey and Mouser, allowing me to get all the bits ordered.

This is a straightforward construction project, thus good as a relaxing break from other projects where I am designing and solving problems. I have soldered IC sockets onto both PCBs, all the small filter capacitors at each chip site, and some of the resistors, based on stock I already had, the rest will have to wait until my various parts shipments arrive.

The trace size and spacing between pads, vias and traces is very small on these boards, offering quite a few spots where solder bridges might be inadvertently created. I will go through the boards visually and with a continuity tester to check for any of these and correct them. The kit did require very good soldering skills and careful assembly, well within my current capabilities but I can see how this could be daunting for many who might have attempted it.

As an update, I received most of the components late Saturday and have the SBC6120 itself up and running. I didn't have one chip, the 6402 (UART), but had an old board that held one. I unsoldered the chip, cleaned up its pins and inserted it into the socket on my assembly. I worked though many of the monitor commands of this kit, but don't yet have a software image on disk ready to use with a PDP-8.
PDP-8 replica - the SBC6120 - after construction
The front panel (FP6120) is missing just a few parts, but I have it almost complete. The LEDs themselves, a rotary 'mode' switch, and some mounting hardware are the pieces that didn't come Saturday. When they show up later this week, I will complete the assembly and test out my new front panel.

FP6120 front panel, waiting for LEDs and rotary switch to arrive

Panel front that will be attached to FP6120 and SBC6120

1401 panel and simulator/emulator 

I have the panel from an IBM 1401 computer, which was marred by too much heat from use of an incorrect light bulb by a prior owner, but I can compensate for the browning/yellowing of that area by using different color temperature LEDs to make a working replica - much later, when the 1130 is completely wrapped up, probably a few years from now.

I think it will make a good adjunct to the PC based development environment for the 1401 that was created by Ron Mak and others for use in the 1401 restoration project at Computer History Museum. Not a high priority but an interesting future project.

Core memory demonstrator

I bought a core memory plane in good shape, originally used in a 2821 control unit for a S/360 mainframe, so that I can build a working core memory sometime in the future as a way to flex some design muscles. Again, this is not an immediate project, just an acquisition on ebay I concluded this week.

Core memory plane to be used in future demonstrator

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