Electronics > Repair
TDS3014 adventures
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james_s:
So I'm still waiting for my boards to arrive from JLC PCB so no movement there. I do however have some other stuff to share that some might find interesting. I decided to play with the DS1742W and threw together an adapter to connect it to one of my FPGA boards. The FPGA is not really doing much other than serving as a patch panel to conveniently route the switches and buttons as well as provide decoded hex displays. This shows selected address, byte to be written, and byte read with the byte read also shown on the red LEDs, convenient for manipulating individual bits.

So I started poking at the DS1742 while referring to the datasheet, I have a much easier time understanding something if I can actually see and touch it vs just studying the datasheet. Anyway I figured out why I've had difficulty programming these things in conventional EPROM programmers, it turns out that in order to modify any of the clock related registers you must first write a '1' into the 'W' bit of the control register, otherwise they are read-only. By default (with a dead/missing battery) it also seems to power up with frequency test enabled, this causes bit 0 of the seconds output to toggle at 512Hz which can cause strange behavior if it gets enabled by mistake. The DS1642 used in the TDS300 series has the same feature and it causes the displayed time to jump around erratically. Anyway I thought this was pretty cool stuff, I can hand program it, setting the time and date and watch the clock run and a setup like this is fully adequate for setting specific bytes like the power-on hours of the scope, no EPROM programmer needed and very little chance of anything getting accidentally corrupted.

As I was messing with this I happened to be looking at the status register when I bumped the DS1744 and the battery flag LED came on! Additional poking revealed that the fault that set me down the road of building a replacement DS1742W is actually a cracked joint down under the epoxy where the nickel strip that connected the original battery is soldered to the PCB. This means I should be able to repair it easily enough and that would have saved me 60 bucks. Since I already bought the parts I'm going to continue down the path of building a replacement and assuming that works I'll share the design so others can use it and keep the original DS1742W as a spare.

Incidentally the PowerCap version of the DS1744 arrived and it is a bare un-potted PCB with a commodity SRAM and DS1744D TQFP IC on the bottom. I don't think Maxim sells the bare DS1748D but it must be exactly what is in the currently made potted versions as well. Wish I could just buy that IC and build a direct fit DS1742W module.

Something else I think might be interesting to do at some point is further reverse engineer the TDS3000 and other instruments that use these NVRAMs and find out exactly what locations are used to store what. It would be very easy to wire up an FPGA board in place of the DSxxxx and use internal dual-port block RAM which can be monitored/dumped/poked live. This would be of greater interest to me on instruments like the TDS300 series that store the cal constants in the NVRAM, I have two of those and a friend has a third that all work fine but display a calibration error on power up. I was able to calibrate the voltage with difficulty but the timing calibration is too finicky for any sort of pulse generator I have access to. Perhaps it would be simpler to disassemble the firmware and attack it from the other direction, I don't know, I'm more of a hardware guy.
james_s:
Finally got the boards I ordered from JLC PCB and built the adapter today. I'm pleased to report that it worked perfectly right off the bat, just plug it in and go. After verifying that it worked, I used the FPGA board I was playing with before to set the power-on hours and now it's buttoned up and finished.

Adapter board gerbers and design files here:
https://github.com/james10952001/DS1744WP-to-DS1742W-adapter

This should work for any application of the DS1742W however you'll want to verify the mechanical fit if you are wanting to use it in anything other than a TDS3000.
giosif:
Congrats on your work and thank you for sharing with everyone else here!
ArcticGeek:
Nice work - glad to see that it worked right out of box!  Thanks for sharing with the community.
daveyk:
Hello James.  Do you have any extra boards that you had made up that you would sell?

Dave
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