Tuesday, May 2, 2017

Progress on Alto disk tool, planning improvement to IBM 1130 display panel, and helping move a 4331 system


Working through the logic analyzer trace line by line, decoding and understanding the microcode of the Disk Word Task and its related hardware, led to the discovery that some of the comments documenting the microcode used numbers that appeared to be decimal but were in fact octal values. 

Thus, when the DWT writes a sector, it begins with 34 words of zero (in octal) which is only 28 words. My spec for writing the sectors had a few such constants wrong. Working through it all, I have an updated spec and am in the process of adjusting my fpga logic to fit that. 

To understand this well, I had to master the Alto microcode and controller hardware so that I know exactly what is happening on the disk. I had a logic analyzer trace while the Alto read a sector, which I used to ensure I knew the exact sequence and timing of the operations.

I didn't have a corresponding trace of a disk write, but I mocked up a copy of the trace, changing each instruction which varies during a write instead of read. It confirmed my newly corrected spec for the disk format and operation. 


On the IBM 1130, a pedestal above the console typewriter contains about 150 incandescent lamps behind a smoked plexiglass faceplate, used to report on the values in key registers and system state information. Any time a bulb burns out and needs replacement, the design of the pedestal turns this into a long, tedious and frequently futile effort to end up with all lamps working correctly. 

Lamps pushed onto PCB boards
Honeycomb that the bulbs must fit into
I conceived of an alternate implementation for the insides that would preserve the incandescent bulbs, SCR control of the bulbs from a 7.5 VAC supply, and lamp test functionality, yet would allow for fast and easy replacement of any future failed bulb.
Boards pushed into the honeycomb - all 16 bulb sockets must align perfectly
I will replace the long shallow PCBs used by IBM. These contain an SCR module and have two pins to push into each light socket. The socket is in turn pushed into the honeycomb. The stiff power wires on the left or right end of the board, plus the 16 yellow signal leads running to the SCRs, make it extremely hard to maneuver the board and all its sockets into place. 

Instead, I will build one wide and tall PCB that is oriented in the plane of the honeycomb (top to bottom, left to right but not front to back). It will have a socket in place for each of the sixteen lamp positions of the existing boards, replicated so that the one new board replaces the six rows of old boards. 

The lamps used by IBM have bare wire leads coming out of the glass envelope. They are inserted into a plastic socket which has two cylindrical metal openings at the rear for insertion of pins. One of the pins for a light is fixed on the front of the SCR module, but the other is attached to the PCB with a looped pigtail wire. The pigtail wire can't be easily pushed into the lamp socket. 

Bare wire lamp inserted in plastic socket
pigtail wires
SCR modules on board
If the wires from the bare bulb extend out of the side of the plastic socket too far, they can short to adjacent sockets, which results in a block SCR module. This module has two diodes as well as an SCR, to provide individual firing of a lamp from the signal coming in by yellow wire on the back, and also bulk firing of all lamps from a 'lamp test' signal fed to all modules simultaneously. 

If the pins are slightly bent, the socket will also be bent. Since the pins must be press fit into the socket but the socket must also be press fit into the honeycomb, the result is a board with 16 sockets all slightly skewed from the position needed to push into the honeycomb. Now imagine the stiff wires that are in place and you see why it becomes quite challenging to fit even one PCB into place, much less 12 that are very close together. 

bulb sockets pressed into honeycomb
Roughly 150 lamps more or less inserted into place
My solution will allow two large PCBs to swing out from the left and right sides respectively, giving access to plug in my new lamp sockets for each failed bulb. The right side PCB, viewed from the angle of the picture above, will have six rows of 16 bulbs stacked vertically, with all the yellow wires you see running to pins on the back of the PCB. It is anchored to the ground, 7.5VAC power and 'lamp test' line at the right edge, those wires acting as its hinge. The left side will hold the remaining lamps. 

Instead of the plastic sockets used by IBM, I will use a two pin header strip, with the bulb leads wrapped and soldered to the pins. A header socket on my PCB will be in each place where a lamp should sit. I can quickly pull off one and put on a replacement just by swinging the PCB backwards. The bulb itself will stand off the PCB because of the header strip and then its own length, so that it will be deep inside the honeycomb when the PCB is pressed against the back of the honeycomb structure. 

Instead of the fairly large IBM SCR modules, I will use modern surface mount SCRs and diodes placed on the rear of my PCBs. I have ordered the parts I think will work, in order to build a prototype and do testing. My method will replace the plastic sockets and narrow PCB boards with their SCR modules, but preserve everything else inside the pedestal. It could be returned to original condition if a new owner wished to do so. 


A fellow collector bought a 4331 system, along with a full complement of peripherals, but had to move it roughly 150 miles to its new home near Grass Valley, California. I agreed to help him move it this past weekend, along with a few other helpers. About 9000 pounds of boxes, cables and parts were involved in the move.
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Loaded truck
The system included the following peripherals:

  • 2540 card reader/punch
  • 1403 N1 line printer
  • four spindles of 3340 disk storage
  • two 3420 tape drives
  • 2821 and 3803 control units
  • four 3278/3279 terminals

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4331, 3340 spindles, 3420 tape drives
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3420, 2540, and 1403


  1. hello,
    do you know if he wants to keep everything? i'm looking for the kybrd for 3278 , do you now if he wants to sell some?

  2. Hi Roderick

    Guy plans to keep all of it, has other 3178/3278 terminals already and is looking for more. Sorry to report this, as i know how hard it is to find 3278 particularly the 2A keyboard needed to operate the 43xx series systems.