Analog ROMs and vector characters - Page 1
Forum section: electronics / projects

author: helius
author_rank: Super Contributor
timestamp: May 10, 2019, 04:37:46 am
content: I was reading about the readout system in the Tektronix 7000 series:http://w140.com/tekwiki/wiki/7000_series_readout_systemThe waveshapes come from an "analog ROM", a custom chip with storage elements made from multi-collector transistors. The metal layers connect variable numbers of collectors to each line. The current from each element goes to an integrator that steers the beam.I happened to see another project for producing vector characters on a scope, which also uses a discrete analog "ROM":http://www.glensstuff.com/fouriersynthchargen/fouriersynthchargen.htmAre there any other systems that use these techniques? With the new availability of "memristor" devices (junctions whose conductivity depends on charge transfer) will there be a new day for analog semiconductor memories?

author: David Hess
author_rank: Super Contributor
timestamp: May 10, 2019, 05:11:40 pm
content: Later, there were compatible variations (three or four) of the Tektronix 7000 readout which replaced various elements on the readout board as they went out of production. The last variation was all digital with a microcontroller but there was an intermediate one where just the analog ROMs were placed with a digital ROM (27C256) and the output was integrated. Full documentation for that one can be found in the 7904A service manual.

author: duak
author_rank: Super Contributor
timestamp: May 10, 2019, 06:37:51 pm
content: "Analog ROM" made me think of I2L - Integrated Injection Logic - invented by Philips in the early 70s. Digital inputs, multiple current sink outputs. Somewhere I have a TI sound generator chip, probably this one:http://www.righto.com/search?q=76477that uses I2L to control various analog functions.In a sort of related question re Tektronix readout generation, I have some older Tek scopes, one of which is a 2245. Is it possible to modify the readout to handle the X50/X500 attenuation factors of the P5205 differential probe? If memory serves, the scopes readout circuitry measures resistance to ground of the ring connection of the input BNC to determine the attenuation factor of the probe.

author: james_s
author_rank: Super Contributor
timestamp: May 10, 2019, 06:47:02 pm
content: That's bizarre, I had no idea they had done such a thing. Seems like it would have been simpler to use digital from the start, it's trivial to convert digital words to analog values. There was a vector monitor test source for repairing early 80s arcade games which used TTL counters to clock EPROMs feeding DACs to generate the test patterns.

author: David Hess
author_rank: Super Contributor
timestamp: May 11, 2019, 02:04:40 am
content: Integrating the result means that the character DAC resolution only needs to be 2 or 3 bits. 74HC574s were used as resistor based DACs. A microcontroller sequences the ROM which explains why this method was not used originally; microcontrollers simply did not exist then.The 2232 series of DSOs do something similar and I think this produces the best looking readout. The 7854, 24xx series, and I think the last 7000 readout design use a raster.

author: helius
author_rank: Super Contributor
timestamp: May 11, 2019, 03:01:16 am
content: The 2232 readout does look very clean. As does its wizard: