vacuum-fluorescent
I salvaged this display unit from an old VCR about 20 years ago. Since then, it's been lying in my electronics box. Now I would like to play around with it on an Arduino. I can't find the datasheet for this VFD online. I've never used a VFD before, so this is completely new ground for me. Any assistance would be greatly appreciated.
If you want to experiment, you can drive a VFD using an Arduino without a controller chip. If you can't find a datasheet for your display it'll take a bit of trial and error.
First, you'll need a couple volts across the filament. This will most likely be the first and last pin. For best results this should be AC, but DC is usually acceptable. The 5v from your Arduino will be too much so put a small resistor in series with the filament, for my display, I used a 47ohm resistor. If you skip the resistor you'll see the filament glow red hot. Since these displays are not designed to be light bulbs, you shouldn't do this for more than a couple seconds, but it might be useful to confirm that you've found the filament. I found that the display was brighter if I hooked up the first pin to ground and the last pin to 5v via the resistor. I'm not sure why this is the case.
Now you probably won't be able to drive the grids with 5v, but depending on your display you might not need much higher. I have a display lit to a decent brightness at the moment with only 7v which I've pulled directly from the supply rather than going through a regulator. (If you can't do this with your Arduino, or your particular display needs 20v or something, you can use two different power supplies by connecting the ground together, though this is not a good idea long term.)
Your display will be broken into separate grids and it will probably be obvious which pins are connected to which since these things made of glass and you can look right inside. So you'll have one pin for each grid and the remaining pins connect to the individual segments that light up.
So here's where VFDs differ from LED displays. In an LED display you'll have a common anode or cathode and the individual segments will be the reverse. In VFDs, both the grids and segments are anodes. So hook up your higher voltage to at least one grid and one segment. Doing this, you can map out which pins light which segment.
This can all be done without an Arduino at all. What you'll notice now is that you can only light up the same configuration of segments for each grid. Not very useful.
You'll need to multiplex the display: light up the grids in rapid succession, changing the segments that are turned on each time. This is where a controller chip can be handy but it can be fun to write using just the Arduino.
If you have enough free pins on the Arduino for the anodes that you need, you can use a transistor on each as a switch for the higher voltage. You can use the analog pins as digital pins as well, just refer to them as pins 14-19.
The best way to multiplex a display is using an interrupt on a timer, so read up on those.
Hope this helps.
Edit
Here's a circuit with a calculator to build a voltage booster for powering VFDs.
[This is a bit of a "brain dump", but I have limited time to try shortening it at the moment.]
My intent was to test the device using a serial-connection from my PC, firstly via a COM port
Go back to that. The VFD datasheet clearly shows it is designed for an RS-232 (or for younger readers, TIA/EIA-232 :-) ) \$\small{SIN}\$ signal. Standard 8-N-1 format, so you can set that on Hyperterm / Putty or whatever terminal software you want to use on the PC.
now via a Platronix USB RX/TX thing
I couldn't find anything with that name via a quick Google search (so please provide a link). However few of the typical USB-to-serial adapters have an RS-232 output; instead they are usually some kind of logic level output (sometimes called TTL output). Don't use that now, for reasons I'll try to explain briefly.
The VFD datasheet is potentially confusing:
I can explain more about the \$\small{SIN}\$ signal shown on the datasheet, if needed. However in short, everything is telling me to use RS-232 for that input.
You also need to have a common 0V reference (i.e. connection) between your PC and the VFD power supply.
The \$\small{SBUSY}\$ signal is different:
I remember reading that some of those cheap logic analysers can have unexpected problems with 5V input signals. Therefore, at least to start, I would use one of your multimeters to measure the idle voltage of \$\small{SBUSY}\$. Since that isn't a valid RS-232 output, I don't want to guess what the datasheet diagram is showing for the idle \$\small{SBUSY}\$ voltage, but for this discussion. let's assume it is 0V when idle.
As each character is sent from your PC to \$\small{SIN}\$, you would expect to see a small "blip" on your multimeter (the measured voltage is not important - we know it is 5V, if it stayed stable for long enough for your meter to display that). You can only send 1 character at a time to the VFD, expect to see a small "blip" on the meter, then you can send another character.
It will be interesting to verify that you see a "blip" on the meter, the approximate duration of that "blip", and the idle voltage on \$\small{SBUSY}\$.
Of course you could replace the multimeter in that experiment with an LED+resistor combination. However for short pulses, I suspect that the meter may give a more visible indication than a very short pulse on an LED. If your logic analyser was known to be usable on 5V signals without an excessive loading, then of course, you would use that - but I can't confirm anything about your analyser.
Brief answers to your questions:
- How, given my limited resources, how should I go about debugging the connection to control this device?
I vote for - get it working via the PC, then move forwards, given that the VFD is intended to receive an RS-232 signal on \$\small{SIN}\$. For your final project using an MSP430, you will either need to generate an RS-232 output or else use the VFD's parallel interface.
The non RS-232 \$\small{SBUSY}\$ is a fly in the ointment, when connecting to a PC, however :-(
- Should I consider trying to build other circuits to debug this one?
- I considered trying to use the ADC of the MSP to make a "ghetto" USB oscilloscope
You could do that, but I am not convinced that doing so, and then having to debug that, will be a "net win" overall. However do keep an oscilloscope on your "to buy" list. It was the first piece of equipment that I bought as a teenager, and I still have it in storage - can't bear to give it away (an old 10MHz Hameg). As you are seeing, without one, troubleshooting gets harder.
- Can my PC (with several RS232 ports, that is very powerful by enthusiast standards) be of any help to me?
As I explained, the RS-232 port can be useful, I believe. I have read about software to make a PC soundcard into a (very limited) oscilloscope, but it's AC-coupled only and would be of limited use here.
- Should I give up without better tools?
It depends how you get on with some initial troubleshooting, as I have suggested e.g. using your PC to send the commands to the VFD. I would not give up yet, but as I mentioned, the lack of a 'scope, is making this much harder.
- What should I look for with my logic analyser?
First, you have to get confident in that logic analyser, and (as with any test equipment) find its limitations. The lack of documentation will hamper your efforts here too :-( If you know that it can be used for 5V signals without excessive loading (which was my memory of an article I read some time ago), then by all means use it to monitor \$\small{SBUSY}\$. However the multimeter will be better to confirm the idle voltage of that signal.
- Is watching the RX and TX line enough? Should I look on the VFD module itself?
There is no data signal out of the VFD, so (depending on whether you meant Rx/Tx from or to the VFD module) you only need to consider one of those two signals - PC-to-VFD. Do you need to monitor it? If you had a 'scope, then yes. You must not connect an RS-232 signal directly to that analyser.
So without a 'scope, you could construct an RS-232 to logic level interface (e.g. based on one of the new members of the MAX232 family like the MAX3232) and feed that to the analyser.
It is impossible to predict whether it is worth you building that now, or waiting until you have specific test results where you can only get the answer with that. And again, after building that, you need to test it and get confidence in that home-brew test equipment, otherwise it just adds extra variables and potential sources of bugs. :-(
- Is there anything "obvious" in the "SBUSY handshake" the datasheet describes that I have missed?
The \$\small{SBUSY}\$ handshake looks very simple. If that signal were at RS-232 levels, it would be connected to the \$\small{CTS}\$ pin on the PC's serial port, and you would then enable "hardware handshaking" in your terminal software but that then creates other tasks e.g. you will likely need to also connect DTR to DSR on the PC, when you enable hardware handshaking.
Since your final objective is to use an MSP430 and not a PC to drive the VFD module, I didn't suggest putting too much effort into the PC-related testing and hence I didn't detail all the steps to interface \$\small{SBUSY}\$ to the PC. However this is certainly possible.
- Am I trying to do particle physics using a stone-club and a burning twig?
A little :-) There are many steps ahead of you, and the troubleshooting plan may need to be adapted at any step, depending on what happens in the previous step. Therefore it is not possible to produce a full "troubleshooting tree" at the beginning, which takes account of all possible results which could occur.
The lack of the 'scope adds uncertainty, and the logic analyser does not replace some of the facilities that a 'scope provides. You might be able to do this project without having a 'scope, depending on what results you get at each stage - although I wouldn't want to do this without a 'scope, as troubleshooting would be so inefficient for some types of problems.
Hope that helps - good luck!
Best Answer
These VFD displays are not so easy to drive unfortunately ! They require a high voltage, in the order of 50 V DC for the anode and a lower voltage but with some current capability to power the filament (those are the very thin wires at the fron of the display just under the front glass). To light up an element it has to be pulled down to ground as far as I know.
Have a look at this page where the author shows how to build a clock using some small VFD tube displays. Althoug their shape is different, the technology inside is the same.