My Bubble's Back and There's Gonna Be Some Trouble

A project log for Magnetic Bubble Memory Makes a Comeback

An Old Attraction to Magnetic Bubble Memory leads to a 1Mbit Board for the SBC-85, 8085 based single board computer

craigCraig 05/17/2020 at 20:520 Comments

Hey-laaaaa. Day-laaaaaa. My bubble’s back

A quick update on the 7110A bubble board that arrived from the PCB fab shop and is now under test. This version of the board uses the much more common 7110A 20-pin version of the bubble.  In addition to the device swap, the board also has more bulk capacitance to meet the voltage decay requirements, a few relatively minor patches were applied, and some cosmetic component placement changes.  All-in-all, nothing that should break the design.

The short version is something is fishy because I cannot write to the bubble; or at least it will not read back what I have written.

I started diagnosing today and this is where I am now–

I can access the 7220 controller and it is giving me good responses to my queries as well as successful FIFO reads and writes.  So at least I know that all is good in communicating with the bubble controller.

As the next step, I demonstrated to myself that I can read the boot loop and the values match the loop table the label. So that tells me that the rotating field is fine, the amplifier is good, and at least some of the signals from the pulse generator are OK.

Next I was able to write to the bootloop register (remember, you generally don’t want to be writing to the bootloop itself) and then read back this data.  That tells me there is successful 2-way serial communcation with the formatter /amplifier.

If this board were for the 7110-1 leadless version to match the prototype, it would be a simple matter of swapping the bubbles to see if I have a read problem or a write problem (or both).   I am fairly confident it is not a read problem since I am getting the boot loader correctly.  So now it will be back to the oscilloscope to check for the waveforms from the pulse generator.

This is a new bubble from a BPK-70A kit that I have had on the shelf for years so I can’t imagine that it is a bad bubble.  However, in the back of my mind I have to wonder if there is any way this device could have lost its seed bubble.  If it is the seed, that will cause some delays since I will need to write some more code and build a bubble generator board.

Next up, I need to check everything from the pulse generator.  If those all check out, then I need to seriously consider that this is a seed bubble problem.

Wish me luck, I will let you know when I find the problem but wanted to give an update on the status for anyone interested.

UPDATE 5-16-20:  Got it.  Details to follow.