Looking for a way to drive unknown semi-complex VFD with an Arduino - Page 1
Forum section: electronics / projects

author: GrandTheftAuto4life
author_rank: Contributor
timestamp: September 15, 2021, 02:34:48 am
content: Hello.I've gutted my old failed DVR for parts since I want to re-use the VFD, but I'm not quite sure about the driving part.As far as I can tell, this is a DC-biased VFD but the part number is unknown to google.However the system schematics is available.The current chip that the display is connected to is some generic toshiba cpu with integrated I/O for the VFD and I would like to swap it out for something generic, preferably some kind of IC or alternatively a discrete switching board (though it's a lot of segments and pins)I've attached the PSU and frontpanel schematics for reference purposes.Presumably +40VE is DC positive and +40VS negative, but I'm not fully sure.As for the wiring, there is:1G ... 12GP1 .. P16I've looked around for possible solutions and the part PT6315 looks like it would work, but there is no library for it and I struggle a lot with protocol timings so I can't really see myself writing a library for this project.I found a raspberry pi project on github for a similar but too small chip:https://github.com/pablogad/libvfdCould someone please help me verify that the display is DC-biased and point me in the right direction to control it?

author: Ian.M
author_rank: Super Contributor
timestamp: September 15, 2021, 03:40:48 am
content: Unless your DVR failed totally dead, its a pity you didn't make some measurements before gutting it. It would have been very useful to know the filament voltage and the -Vkk rail voltage. They aren't given on the schematics and if you cant find them by thorourgh reading of the service manual some very careful experimentation will be required to find suitable supply voltages.The original VFD driver operates between +5V and -Vkk, and I estimate the -Vkk supply may be somewhere around -30V, with the filament biassed 7.5V positive of that at one end. The other end is fed from a floating DC supply. From their pinouts, the two possible displays have 16 segment or symbol anodes and 12 digit grids (some of which may be for symbols not digits) There are also 13 NX pins on the schematics, which I suspect stands for 'no connection'.Microchip have a number of HV push-pull driver chips that can run VFDs. See:https://www.microchip.com/en-us/parametric-search.html/442I'd probably choose theHV518which is a serial interface 32 channel VFD driver. Hook it up to the Arduino's SPI pins and write a timer ISR to handle the multiplexing, sending 4 bytes each time, two with one of N bits high to enable one of the digit grids, and the other two with a 16 bit segment pattern. From your main program's point of view, it would be effectively memory mapped, one bit per visible segment or symbol. You'd also need a const PROGMEM array to store font data to translate ASCII into a 16 bit segment pattern.Although the display was used with a DC filament supply, I'd probably go for a capacitively coupled AC one, just because it makes the biassing much easier. Seehttps://www.eevblog.com/forum/projects/showing-my-vfd-psu/(Rolo's image links are in my reply #10)

author: GrandTheftAuto4life
author_rank: Contributor
timestamp: September 15, 2021, 06:51:41 am
content: Quote from: Ian.M on September 15, 2021, 03:40:48 amWith the failure being a domino effect, it wouldn't turn on properly even after replacing 4 capacitors and 2 diodes that failed.VFD didn't turn on at all in standby (it's supposed to be dim) and it shuts off while the firmware is loading.I tried to locate the issue but I couldn't find it. With my luck it's probably one of the mainboard BGA chips that's failed.Thus it went in the recycle bin.Quote from: Ian.M on September 15, 2021, 03:40:48 amJudging by the zener diode and the -FL connection near the -VKK pin, I'd guess it should be -31VAnd yes, the NX pins have no connection. (they don't even reach down to the pcb)Quote from: Ian.M on September 15, 2021, 03:40:48 amAlright. As for the segments, it's 10 characters, 14 segments, no per-character colon or comma and a bunch of other segments such as a "hdd space indicator" (it does have colon and comma but those are at two fixed locations)This looks promising though, thank you.Edit: added picture

author: james_s
author_rank: Super Contributor
timestamp: September 15, 2021, 07:30:10 am
content: That looks like a fairly simple display actually. Are you sure the IC on the board is a microcontroller that drives the display directly? Usually there is a special VFD driver that is essentially a big shift register that you just clock bits into to activate the segments. If you can't figure out how to use the IC that's there, Supertex make a line of HV shift registers for VFDs, and I think Maxim does too, or whatever they are now, I can't keep track of all the mergers.

author: GrandTheftAuto4life
author_rank: Contributor
timestamp: September 15, 2021, 07:34:39 am
content: Quote from: james_s on September 15, 2021, 07:30:10 amYeah, it's a microcontroller with integrated vfd driver, you can take a look at it's datasheet here -https://cdn.datasheetspdf.com/pdf-down/T/M/P/TMP86CM72FG_ToshibaSemiconductor.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: September 15, 2021, 07:55:27 am
content: Supertex is now part of Microchip Technology. Maxim has just been acquired by Analog Devices. Hopefully they'll manage to render the sci.electronics.design maxim: "Friends don't let friends buy (from) Maxim!" obsolete.Looking at the visible control grids on the O.P's display, I suspect that the decimal point and both dots of the colon are wired as per digit anodes for the two digits that have them. How the symbols are mapped in the last two grid cells is anyone's guess, though there aren't enough anode pins for all the segments of the spinning disk to be individually addressable as it shares a grid with the HDD space indicator.

author: GrandTheftAuto4life
author_rank: Contributor
timestamp: September 15, 2021, 09:29:43 am
content: Quote from: Ian.M on September 15, 2021, 07:55:27 amI think they are grouped 4x4. If i counted correctly there is 16 segments for the spinning disc

author: james_s
author_rank: Super Contributor
timestamp: September 15, 2021, 05:55:40 pm
content: I'm going to guess that the spinning disc symbol consists of pairs of segments opposite each other tied together. Determining the pinout of the display itself ought to be fairly straightforward, just connect power to the heater and then try about 20V between the heater and various anode and grid pins. If you can identify the pins for the grids visually you can tie all of those to the same positive supply you're using to check the segments. Very little current is required so use a resistor to limit it as you probe the various pins. To work out the heater voltage, set a bench supply as low as it will go and then very slowly crank it up until they just start to glow visibly then back it off a tad. Typically they should glow very faintly reddish in a darkened room.