Saturday, May 13, 2017

IBM 1130 panel schematic, Alto restoration progress


I decided to attach the circuit drawing for the replacement lamp driver circuit I am building, noting its similarity to the original IBM circuits although superior from a mounting and service standpoint.

Lamp driver circuit - lamp test switch shown at bottom

IBM 1130 circuit by comparison


Still not booting from the cartridge after I write it. Did read of the sector and captured on a scope. Now for extremely tedious counting and measurements to see what/if I did something wrong.

I centered the scope on some key pulses, such as the one bit that comprises a sync word, to get precise timings relative to the sector start, then other sets of pulses were interpolated from the grid marks on the scope face so that I can determine the timing with reasonable precision.

The clock pulses are in a train on the other trace below the data bits, one every 600 ns in a sector that 3,333 us long. That is a LOT of clock pulses and for long strings of zero words I have to count them. Each word is 16 bits therefore 16 clock pulses per word. For example, the separation between records in a sector is 10 words of zero plus a sync word with a single bit on. That is 176 clock pulses that have to be carefully counted.

Then, in the data words of a record, I have to carefully count clock pulses to divide out the data bits into words, before checking them against the file contents I tried to write to the cartridge. Checking 326 words in a sector involves accounting for 5, 216 clock pulses.

I spent the day Saturday capturing and recording the timings of the 1 bits, but was only beginning the checking of clock pulses to align them into words by the end of the day.


Yesterday we made progress on several fronts and dug into diagnostic information on others. The Digibarn machine was further cleaned and examined. The monitor that came with it is built for a Dorado, a later machine. It has a 6502 microprocessor on a board inside that multiplexes the monitor, keyboard and mouse signals over a bus using seven differential pairs with fast ECL drivers. That is very different from the connectors used with the Alto. Fortunately, we can intercept the signals from a connector inside the monitor and wire up our own cable adapter.

The capacitors to replace inside the power supplies have not yet arrived, so we didn't make any progress on that part of the Digibarn machine. Al Kossow warned us of a tantalum capacitor that often fails in the supplies, which we will also check before finishing the restoration of the supplies.

Ken's network tool passed several milestones, successfully handling echo protocols, telnet connections and ftp connections. The ftp protocol is different from the more commonly known protocol that comes with TCP/IP systems, but has similar goals. The client connects to the server properly, and command strings are mainly recieved and parsed properly, but the unit is not yet executing those commands.

With a bit of tweaking, we had the LCM ethernet bridge operational, which will be important to link together groups of Altos as we more are made operational. We did some network booting and communications, but didn't explore the file server or other utility functions. 

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