Thursday, May 18, 2017

Mostly testing and data collection tasks on the Alto


I stopped by the workshop in Marc's basement and did some testing with my logic. I discovered a problem in the way I load the shift register that serializes each word to be written to the disk drive. I am clobbering the data in the shift register before it has been written fully out, in some cases, which manifests itself when the sync word (x0001) last bit '1' does not make it onto the disk.

The challenge here are several independent processes with strong timing requirements that have to interface with my overall logic to write out a record. Another timing dependent activity is RAM access, taking multiple fpga cycles to complete, in order to fetch the next data word.

One machine is emitting clock bits every 600 ns and at the 300 ns point, midway in that cycle, it will emit either a 1 or 0 data bit. The serializer must shift out a bit to set it up for the first machine. The serializer counts down the 16 bits as they are shifted out, then emits a signal to get the next word.

My overall record-writing logic must load words when the serializer requests them, but also has to have read the data word from RAM in advance to allow it to feed the word on demand to the serializer. I issue the load request for the serializer as soon as the RAM read is complete, but that is before the prior word is fully shifted out since I have to do the reading in advance.

I am refactoring the logic again to get this cleaned up and will then have to test next week, after Maker Faire is over since many of us will be doing duty there.


In addition to the data capture I pursued, we worked on a few other tasks. I upgraded the ethernet bridge machine (on loan from LCM) with the latest code they sent. We made use of it to boot over the network and explore many functions and capabilities. The one we failed at was achieving a copydisk over the network, to read a cartridge in the real Diablo drive in the Alto and copy it to the bridge machine which is PC based.

Ken worked with his ethernet tool to check out and build up his protocol support. His code begins the network booting sequence for the Alto getting back the specific image request but is not yet able to send the entire image. He is able to connect as ftp users and ftp servers, but something is still going awry with building command lines the server will accept

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