| Electronics > Repair |
| TDS3014 adventures |
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| ArcticGeek:
--- Quote ---DS1558 won't work, the register map is different, it has a lot more features and some things are moved around. Maybe there is another similar part that is the same as used in the DS174x parts. --- End quote --- @James Yea, I was aware of the difference between the DS1742 and DS1558, but I believe it will still work. You will notice that address range XXX8 thru XXXF are identical between the two parts, its just that the DS1558 has some extra regs from 0 thru 7. Since the DS1558 has a full 512K of address space and the SRAM is only 2K, you can play a trick on the address bits of the DS1558 to fool it. The DS1558 will forward the chip select to the SRAM only if the address lies outside of the range of 0-F. If the address lies within the FFFFF0 and FFFFFF range then the DS1558 does not assert CS to the SRAM and it responds on the bus. I believe the following trick will work: The address pins A[0:10] of the SRAM are wired to the scope address pins A[0:10] as they should be. Address pins A[0:2] of the DS1558 are wired as they should be to pins A[0:2] of the scope address bus. Address pin A3 of the DS1558 would be tied high to 5V. Addr A[4:10] of the DS1558 is connected to A[4:10] of the scope as well. But address pin A11 (or any other higher address for that matter) of the DS1558 is connected to A3 of the scope. In this way, anytime A3 is low (such is the case in address range 0-7), the DS1558 will not recognize it as a valid range for it to respond on the bus and will forward the chip select to the SRAM. All of the other address bits of the DS1558 would need to be tied high to 5v. Doing it this way, the DS1558 would respond on the bus anytime the address was in range of 7F8-7FF, and its register map would be identical to the DS1742. However, when the scope accesses address range 7F0-7F7 the DS1558 would see this address as 7FFE8-7FFEF and would not respond on the bus and forward the CS to the SRAM. This would have the effect of making the memory map of the DS1558 identical to the original DS1742. The DS1558 regs at 7FFF0-7FFF7 are never accessible, but that's okay because they'll never be used anyway. I haven't looked at this in super detail, but I'm fairly sure it would work. I hope my explanation makes sense! |
| james_s:
Hmm ok so looking over these maps again, they're *almost* identical except for one thing, BF the battery flag is at 7FC on the DS1742 and 7FFF3 in the DS1558. In this particular application I don't think that matters since as far as I know these scopes never look at the battery status. I might have to build something at some point just to see if it would work, I looked around and it seems the DS1558 is the only similar standalone RTC IC that they still make. I've also pondered various solutions with a CPLD or small FPGA which could easily do any kind of address translation desired and it should also be possible to interface a modern serial RTC to the bytewide format. It might even work to use a serial FRAM based EEPROM combined with block RAM in the FPGA to eliminate the need for the SRAM and battery management. I wonder if the still available TDS3054C still uses the same obsolete timekeeping RAM? |
| ArcticGeek:
Yea, I had noticed the battery flag as well, but I would be very surprised if that bit is ever looked at in the Tek firmware. To answer your question about the TDS3054C, yes, I believe it probably has the exact same DS1742W part. I have a TDS3014C and it has the DS1742W part. I haven't replace it yet, but as I recall it is getting close to the 10-year mark and will be due for replacement fairly soon. |
| james_s:
Well this scope is fairly old, the DS1742 was date stamped '99 IIRC so you might still have some time. Mine was dead enough that the RTC wouldn't keep time but the RAM would hold its contents, that surprised me since the battery measured 0.6V. Anyway assuming my adapter works I'll send you one of the boards if you want, I had to get 5 of them and I'm unlikely to ever need more than one. I believe the DS1742W went out of production in 2018 which makes me wonder if Tek is still manufacturing these scopes or just selling existing stock. I was a bit shocked to see they still retail for over $20k new. That's a lot of money for something with a 10 year time bomb soldered to the mainboard. Or you can of course just hack in a new battery, just don't use a programmer that will give the thing 5V to dump the contents. I'm thinking I'll just make up an adapter so I can use the switches, buttons and displays on one of my FPGA dev boards to just hand program the 4 bytes that store the on-hours. Everything else can be set through the menu in the scope. |
| ArcticGeek:
Yes, I would definitely be interested in getting one of your PCBs when you get them. I'll send you a PM for the details - thanks much! My TDS3014C is fairly old now, too, maybe around 10 years old I'm guessing. I bought it on Ebay about 5 years ago and it wasn't new then. I agree that $20K for a TDS3054C today would be a bit of a ripoff, these scopes are great for hobbyist use but a bit long in the tooth for professional use. I don't know why Tek continues to use these Dallas/Maxim RTC chips that aren't replaceable. They started to use them in the 2465B scopes and have continued since then. I haven't opened any of their brand new scopes, but I know that even the higher end TDS5104 and TDS7104 used these things. |
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