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content/hp34970a_3.rst
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| 1 ========================================== | |
| 2 HP 34970A Data Acquisition Unit - part 3 | |
| 3 ========================================== | |
| 4 | |
| 5 :Author: David Douard | |
| 6 :Category: Electronics | |
| 7 :Tags: HP, 34970A, DMM, repair, test equipment | |
| 8 :series: HP 34970A repair | |
| 9 :series_index: 3 | |
| 10 | |
| 11 | |
| 12 As explained in `the previous post in this series <{filename}/hp34970a_2.rst>`_, I've started to sniff the serial | |
| 13 protocol between the main board and the display panel so I can replace | |
| 14 the failed VFD by an OLED or TFT display managed by an arduino or | |
| 15 similar. | |
| 16 | |
| 17 Protocol | |
| 18 ======== | |
| 19 | |
| 20 The serial communication takes place at a rather unusual speed: 187.5 | |
| 21 kbits/s, otherwise, a standard 8N1 scheme. | |
| 22 | |
| 23 The protocol consist in a transmission frame with a start of | |
| 24 transmission handcheck, some commands with arguments, and an end of | |
| 25 transmission packet. | |
| 26 | |
| 27 Each transmitted character is acknowledged by the receiver, either by | |
| 28 a ``<0x99>`` (for the initial handcheck) or by a ``<0x00>`` for the | |
| 29 other ones.. | |
| 30 | |
| 31 I've only probed the CPU->DP communications. I don't really care the | |
| 32 other way fr now since I plan to keep the original 80C51 so it manages | |
| 33 this protocol and handle the keyboard. | |
| 34 | |
| 35 In the direction CPU->DP, we can find: | |
| 36 | |
| 37 - ``<0x66>``: start of transmission, must be acknowledged by a | |
| 38 ``<0x99>`` by the receiver; this start of transmission is then followed by a "command": | |
| 39 | |
| 40 - ``<CMD> <NCHAR> <CHAR1> ... <CHARN>``: the command byte is followed | |
| 41 by the numer of characters sent as argument, then the characters | |
| 42 themselves. | |
| 43 | |
| 44 - ``<0x55>``: end of transmission. | |
| 45 | |
| 46 | |
| 47 I've only identified 2 commands for now, but these are the most | |
| 48 important ones: | |
| 49 | |
| 50 - ``0x00``: send the chars to the main display | |
| 51 - ``0x0C``: send the chars to the secondary channel display | |
| 52 | |
| 53 | |
| 54 Quick prototype | |
| 55 =============== | |
| 56 | |
| 57 With this very incomplete reverse ingeeniring of the communication, I | |
| 58 have enough meat to try a quick prototype. | |
| 59 | |
| 60 I've used an Arduino Uno and a cheap 3.2" TFT color display. As I | |
| 61 wanted to have my serial protocol to debug the code for the beginning, | |
| 62 I've used the SoftwareSerial_ library for this prototype. | |
| 63 | |
| 64 The result is this beautiful clean setup: | |
| 65 | |
| 66 .. image:: {filename}/images/hp34970a/poc_display.jpg | |
| 67 :alt: A very quick proof of concept of an arduino based display replacement for the HP34970A. | |
| 68 | |
| 69 | |
| 70 As you can see, it works: I have the main content od the VFD display | |
| 71 on my TFT module! | |
| 72 | |
| 73 .. image:: {filename}/images/hp34970a/poc_display_mux_off.jpg | |
| 74 :alt: The FTF display showing the main area and the current channel area. | |
| 75 | |
| 76 .. image:: {filename}/images/hp34970a/poc_display_vdc.jpg | |
| 77 :alt: The same with the channel 209 configured as VDC, 6.5digits. | |
| 78 | |
| 79 | |
| 80 The job is far from finished however: I still have to figure out how | |
| 81 the "annotation" flags are encoded and display them, and most | |
| 82 importantly, I have it make it work with the hardwre USART of the | |
| 83 mega328. This seems to mbe necessary because I have glitches | |
| 84 sometimes. But I'm very happy it worked almost at my first try. I was | |
| 85 quite surprised since the baudrate is above the maximum officially | |
| 86 supported one (115200). | |
| 87 | |
| 88 I'll release the code of the prototype any time soon. | |
| 89 | |
| 90 .. _SoftwareSerial: https://www.arduino.cc/en/Reference/SoftwareSerial |
