HM8042 curvetracer - Page 1
Forum section: electronics / testgear

author: free_electron
author_rank: Super Contributor
timestamp: August 22, 2025, 04:06:20 pm
content: i'm trying to figure out how this thing works. ( especially the cursor functionality )Anyone have one and care to dump the ROM ?

author: Kean
author_rank: Unknown
timestamp: August 22, 2025, 05:24:22 pm
content: I was going to suggest contacting member OZ2CPU who did a video on this unit, but I see you have already commented on that video.Hopefully he can help. It was a pretty neat implementation for 30 years ago.

author: free_electron
author_rank: Super Contributor
timestamp: August 22, 2025, 06:12:12 pm
content: Quote from: Kean on August 22, 2025, 05:24:22 pmAlready did. he no longer has the unit.i figured most of it out. i want to "upgrade" the principle to some newer parts (OPA547 output driver, different opamps, ditch the a/d converter and use something like an atmel or stm32 with built in adc as controller + lcd display. that a/d is unobtainium now...)The machine has some interesting tricks like the display marker. i can't quite figure out how they did that. there is some timing between the CRS and RET signal. one creates a "glitch" , the other blanks the trace (i think).it's a play in the timing.I suspect the machine doesn't measure everything "at the same time"For the selected trace they measure voltage in first sweep, current in second sweep. There is a sequencer at work. The sync signal from the sweep generator starts the state machine. The processor launches a timer and waits a specific time (selected by the encoder). when the timer expires it sends a pulse into CRS to create the "dimple" in the trace and triggers the A/D to take a sample of what they want to see. do this 4 times and you have everything to do the calculation.The biggie with this machine is the power limit. they use an analog multiplier to check voltage and current and abort the sweep on overload. nice feature.i recreated most of the circuitry ( the service manual contains big mistakes ! ) .if i could find a unit on ebay it would make my life easier. but they are very scarce.... the version with the built in scope tube is easier to find but horrendously expensive.the rom dump + a disassembler and 8051 simulator should show some insight. worst case i simply replicate the 8031 circuitry to try it out

author: Kean
author_rank: Unknown
timestamp: August 22, 2025, 06:56:46 pm
content: I just browsed the schematic very quickly, and I didn't really understand what the CRS and RET signals are doing, nor the sweep generator circuit in general. Too much going on in the schematic, and other things I need to do.I would have expected that when cursors are enabled, the sweep is just paused for a moment on the one selected trace at the highlight position so that the spot is brighter. Maybe the CRS signal is pausing the sweep generator based on a CPU timer. Based on the name I'd guess that RET is to return the sweep to the start (left of screen), for general sweep timing or should any parameters are exceeded. Or it is blanking the trace by moving it off the screen as there is no Z output for blanking which would otherwise be needed.Obviously the circuit can be a lot simpler with a more powerful processor and a fancy display with digital interface, but yes you'd want to try incorporate the limiting circuits/features.

author: Kean
author_rank: Unknown
timestamp: August 22, 2025, 07:02:29 pm
content: Based on his reply to you on the video, he seems to still have the unit and has an EEPROM programmer that he wants to get working so might still be a chance he can read it.Quote

author: free_electron
author_rank: Super Contributor
timestamp: August 22, 2025, 07:53:12 pm
content: Quote from: Kean on August 22, 2025, 06:56:46 pmThe schematics are convoluted. i redrew them and unwrapped. much simpler to understand then.Sweep is not paused. The microcontroller is a slave to the sweep generator. the 555's are not used as timers, just simple flipflops. Sweep start and end is handled by two external opamps. They make a current source with a jfet to charge a capacitor.when the sweep starts they send a sync signal to the micro to start the timers in the 8051.They create a dimple in the vertical signal by altering the steering voltage.All the control micro does is wait for the sync pulse, delay by adjustable time x ( set by rotary ) and then trigger the a/d to sample.it can display the a/d reading it took as Ic/Id Vb/Vg Ib/ig and calculate gain and slope between two points on a trace or delta between traces.The micro doesn't "control" anything (apart from the dac to set vb/vg/ib/ig and step generation) the whole system works based off the sweep generator.The oscilloscope doesn't even need to be calibrated horizontal or vertical as the cursor takes the measurements based off the marker delay from sweep start.It's a clever machine. (remember this thing is old ! 80's old, that's 40years !)Now they make curve tracer with a micro and some pwm signals and the micro controls everything.This old machine has a charm.

author: Kean
author_rank: Unknown
timestamp: August 25, 2025, 06:14:30 am
content: Thanks for that extra detail. I wonder if they did it this way to get better results with the scope XY mode without blanking.I recall doing a design in the late 80's using 6502 with (I think) 6-bit DACs and XY mode - just a zero-budget hobby project while at Uni.