| Products > Test Equipment |
| Some old school instruments showing how it's done (HP 3325A and Fluke 8506a) |
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| SilverSolder:
"Just having small sections of interrupt code rather than vectors," Than one got me too. - On the plus side, it isn't full of weird segment registers etc. - it seems very 'fundamental' in how it works... |
| joeqsmith:
The segmented memory was something I was used to from my first computer. It predated the 8080 and the CPU was mostly made of 7400 logic. You had 2K of core which was shared between the display and programming areas. To get around the small area, you would have a small program that would load in segments from the floppy and run them. Jumping back and forth. :-DD The 8088 was a big step up. Looking at the code in RAM starting at address 0x112 (0x4112), it appears they set the first location to 0xC& (RET 0) or reset. Interesting is I had noticed if I sent data to the meter over the RS232 port while the meter was booting, it will reset and I could actually hold it in reset forever. I think I now know why. Once the meter boots, they appear to have a mechanism to detect that the RS232 port should be active and then they reprogram this area of RAM. They can certainly pole the communications board to see if there is incoming data. This seems to be what triggers the meter to reprogram that area. Wow ... The more I look at this vintage hardware, the more I want to hear from the team who designed it. |
| SilverSolder:
Perhaps it is able to do something cool, like load an external program into RAM over the serial port, on startup? - like a monitor/manipulator for testing / debugging / special applications? |
| joeqsmith:
I had wondered about it having a debug mode of sorts that allow them to display/change memory and such. If there was, I would have thought the commands would all be ASCII and we would see some visible signs glancing through the hex file. Still, it could be there. I keep thinking 1/60Hz * 8 or a 2.08ms interrupt. In the fast async mode, it averages out to 25.2ms. I just keep thinking there is a /13 that checks that UART's transfer flag. I can tell you that when I initialize the meter, I see the ASCII commands show up a few bytes after the RST4 area in RAM. It also looks like there is a fair amount of RAM that is not being used, at least in my case. I wonder if they use this as a very large buffer when sending in the faster modes. |
| dietert1:
In section 3 of my Fluke 8502A manual they give some technical details about hardware and firmware. There is a detailed explanation of direct and indirect backplane addressing, with the front panel as an example. Also they explain about the timer interrupt (8 times mains frequency) and how the firmware assigns these interrupts to various tasks in a round-robin pattern. Except the ADC has a higher priority and gets half of the time. It's a sophisticated machine - certainly the 8505/6A even more than the 8502A. Anybody has the schematic diagrams of the serial and parallel communication interfaces? In my Fluke 8502A manual it says "Page left blank intentionally". I'm having difficulties with the incomplete decoding of backplane addresses (IC0..IC6). There should be a rule to avoid confusion, but until now i didn't catch it. When wiring up programmers and analyzers to the isolated logic in the DVM, please be aware of a 20 V offset between guard and logic ground. Better leave guard on the front panel unconnected. Otherwise all this may be just a sophisticated way to kill a working instrument.. Regards Dieter |
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