LGP-30 Memory Drum Update

Christian Corti cc at informatik.uni-stuttgart.de
Wed Jan 4 06:05:23 CST 2017

On Tue, 3 Jan 2017, Jon Elson wrote:
> So, does the LGP-30 have permanent storage on the drum, or is it erased by a 
> magnet at every revolution?

Yes, the three timing tracks, and the upper half of the instruction 
counter(!). The upper half containts the number of the following 
sector and is used to locate the desired sector (either instruction or 

> The G-15 modeled the drum just like a mercury delay line, it had a write 
> head, a read head and a bar erase magnet on every line, so there was no gap 
> for the write head to turn on and off between words.  The FF that drove the 
> write amp normally recirculated the data from the read head, except when a 
> word was being written.  Each drum track had a long line (for program and 
> data) and a short line. Most of the short lines held 4 words, so they were 
> quickly accessible.  One short line only held one word, that was the 
> accumulator.

The LGP-30 is similar, but the timing and long tracks (i.e. main memory) 
are not recirculating. Hence the reason why a word is limited to 31 bits, 
although the sector holds 32 bits. The last bit must be zero to reset the 
read circuitry for the next sector.


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