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Understanding a circuit for serial communication - CRT monitor

Discussion in 'General Electronics Discussion' started by --james--, Nov 21, 2016.

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  1. --james--

    --james--

    8
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    Nov 19, 2016
    Hi, I have a CRT monitor that has a small circuit board for controlling monitor adjustments. Basically, this board holds 5 switches and 2 LED indicators. It communicates with the monitor using some type of serial protocol. I could really use some help understanding this circuit. Ultimately, I am trying to interface with the monitor using a serial port. By "interface," I am talking just about the control protocol, not video. Video is through regular VGA. Unfortunately I lost the board while building a project, and I need some way to control the monitor.

    I've attached 3 pictures of the board. Sorry there is a little bit of glare in them. I know it makes it difficult to read some of the labels. The board is really well labeled, which definitely helps. I'm a mechanical engineer, so please forgive my spotty understanding of this circuit. I'll try to sum up my investigation so far.

    * There are 6 lines: 2 are ground, 1 is +5V, 2 are for the LEDs, and the final one is labeled "KBD1." I assume this is a uni-directional serial data line. The monitor sends the LED commands on dedicated wires, but the control board sends commands via the KBD1 line.

    * There are 5 buttons: power, up, down, 1 and 2. The power button just turns the monitor on and off. The up and down buttons move up and down through the on screen menu. The 1 and 2 buttons are context sensitive, but generally 1 is OK and 2 is cancel. Pressing the 1 button brings up the menu. All of the monitor control functions like geometry, color temp, brightness/contrast, and de-gaussing are in these menus. There are no other buttons on the monitor.

    * There are two transistors and one diode, and all the of other components are passive RLCs. There are no ICs. From this, I assume that the serial protocol is a pulse length encoding. Since there are 5 buttons, I imagine there should be 5 commands. It seems that the KBD1 line is normally high, then when a button is pressed is, it goes low for some time determined by the resulting RLC circuit. Does anyone know if this is some sort of standard protocol? I don't know the values of the inductors or the capacitors, so I can't really analyze the circuit to see what the time constants should be. Honestly, even if I had all the values, I'm not confident that I could calculate it correctly.

    If anyone is willing and able to share any more insight into this circuit, I would greatly appreciate it.

    Regards,
    James

    DSCN2362.JPG DSCN2363.JPG DSCN2364.JPG DSCN2365.JPG DSCN2020.JPG
     
  2. 73's de Edd

    73's de Edd

    3,121
    1,315
    Aug 21, 2015
    Sir --james-- . . . . .

    You did not give me good enough non reflective photos of the board to be able to fully "read" it.
    But from what I can make out . . . .
    I see a pair of steering diodes and some resistor sets, maybe 2 transistors, along with associated switches.
    What I see being done is your switches are engaging a resistor pair, such that a voltage division is created
    from that 5V supply voltage .
    That is being a feed back to the instrument proper such that a comparator /comparator set detects that level and engages its proper digital function/variant or machine function if being analog and pre- digital technology days.
    Visualize a voltage spread of 1V - 2V - 3V - 4-V. as being easily differentiated, recognizable and separate information levels to be sent back to the units voltage comparator circuitry, which then initiates the desired actions / functions, in relation to the level of voltage being sent back.
    To somewhat confirm this, fix your missing pushbuttons, hook this board up into the system and stab one DC meter probe into ground and initially monitor SBY BLUE wire while you play with all the pushbuttons produced variants and watch that lines voltage changes produced . Changing ?
    Keep the same ground and NOW monitor the RED KBD1 wire, and do the same procedure, with this more likely being the right info buss .Changing ? aren't they ?

    MAN . . .have they got some abnormally weird color coding assignments on that wiring to connector cable.

    Switch assignments:

    Far right power on off switch, next closest, being a function or mode switch along with the very first switch .
    The second and third switches being the variable up and down adjustments.


    73s de Edd
     
    Last edited by a moderator: Nov 21, 2016
  3. --james--

    --james--

    8
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    Nov 19, 2016
    73s de Edd

    Thanks for the response. I hadn't considered that this circuit could be an analog circuit with the switches just acting as voltage divider. I'll try to look at it from that perspective also.

    The board is from a mid-late 90's CRT monitor manufactured by LG. It is now in a custom cabinet for an arcade game.

    As for the bad glare in the pictures, I do apologize for that. I wish that I had better ones, but these are all I have to work with since I lost the board. For the same reason, I also can't do any further tests on the board. Trying to replicate the functionality from just the pictures is a challenge. I wish that I had taken better pictures, but I never would have imagined that I would need to replicate this circuit.

    I'm not sure I follow the steering diodes. The two black cylindrical components are definitely marked as inductors L101 and L102.

    Also, for the transistors, I am now about 90% confident that they are only involved in the operation of the diode. The LED and STBY lines turn the transistors on and off, which then power the Power and Standby LEDs.

    Again, thanks for the help.

    James
     
  4. 73's de Edd

    73's de Edd

    3,121
    1,315
    Aug 21, 2015
    So this monitor was being tucked away inside the display with the only function having to be responded to was its power on and power off, with no need for menu adjustments.
    I don't think that I ever ran into a LG monitor back in the CRT based display days.
    In the 1990's, I do believe that LG, Goldstar and Zenith were all sleeping in the same bed of which Zenith would've been the better known unit to me, by virtue of their first incorporation of flat screen CRT's .The flat screen aspect was being relevant to the FLAT glass faceplate of the CRT, and not the perceived misconception of some novices, that it was being one of the new , mere inches thick, new generation, big screens of our now era.
    Look over near the power switch and you will see two diodes configured with a common cathode and one series diode down below them.
    It would have been a challenge to have read the one quarter or one tenth watt resistors on that original PC Board .
    Come back with that units model or chassis number where we can see if there is still a schematic floating around that would give us the components and their values on that small board build up.

    73's de Edd

    .
     
    Last edited: Nov 21, 2016
  5. --james--

    --james--

    8
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    Nov 19, 2016
    Your comment about various companies being in the same bed was a breakthrough for me.

    All of the markings on the inside of monitor say LG, but I found an old picture of the monitor case and it has the ViewSonic golden finches. This helps a lot because I was able to find the model number by browsing google images.

    Model: ViewSonic E790

    The chassis is stamped with model 4814TKK083, but this would not have been visible in the original mounting.

    I still haven't been able to find schematics, but I feel a lot closer now.

    cheers.
     
  6. --james--

    --james--

    8
    0
    Nov 19, 2016
    Here is a picture of what the buttons originally looked like. This is from google, not my picture viewsonic-e790-19-svga-crt-monitor-23.20.jpg
     
  7. 73's de Edd

    73's de Edd

    3,121
    1,315
    Aug 21, 2015
    .

    Sir --james-- . . . . .

    WHASSAMATTAYOU . . . . . . .where you go o o o o o o ?

    I was expecting you to be back to this original post.
    Here you are drifting off to oher forums.
    . . . . . . oh behold ye of little faith . . . .

    Got your breadboard at the ready ?
    The dual diode on the board . . .the WAY they laid it out on the board marking,was actually being a dual LED, as seen, when you now consult the schematic.

    As you can see, it is just as I had perceived, there is the 5 VDC reference,and it being fed to the .
    R108 resistor and then it makes a voltage divider node at the KBD line and the option of weighted resistor values at each of the tact switches gives a different voltage level to the KBD line to send to the analog to digital converter at the input of the system U/P.

    CONTROL BOARD SCHEMATIC:


    [​IMG]


    73s de Edd
     
  8. --james--

    --james--

    8
    0
    Nov 19, 2016
    Holy &^%(*

    Miracle worker. Sorry I was away from this thread, but I was doing some traveling for the holidays. You really have helped me out a lot with this schematic. I appreciate it immensely. This saves a lot of work from the trash heap.

    With this I hooked up a breadboard and a few components and was able to turn the monitor on and simulate the button presses. It's unwieldy as is, but perfectly functional. Should be no problem to re-create a proper control circuit now.

    You were totally right about sending analog signals on the KBD1 line. Works like a charm.

    Thanks again.
     
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