TTL comms forever or recent?

LK/ELK/VK/PK/OK/MX/GLK/EGLK/GVK/GLT Series

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dannysauer
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TTL comms forever or recent?

Post by dannysauer »

I bought a Matrix Orbital VFD back in about '98 to install in my car. Since I'm selling the car, though, I'm taking the in-car mp3 player out and reusing the display and computer for differnet things. I'm planning to drive the display with a PIC midrange MCU. Now, I have some Max232s laying around, so I guess I could get the RS232 serial going, but if the thing supports TTL, that's be several fewer parts and generally easier to deal with. However, the connector isn't really labeled to imply that I can do that.

I don't remember what model it is, but I'm pretty sure it's a 20x4, and I know it's a VFD with the keypad interface. Before I go trying to drive the display using the TTL setup, I'd rather like to know whether or not that's even supported on one this old, or if the TTL capability has always been there. If there's a cutoff, is there a way to tell from the serial number or something? Also, something that's not entirely clear to me from the docs - is the communication the same over the TTL as over the RS232 serial, just at different voltage levels (and with rx/tx on one pin), or is there more I'd need to know?

Thanks!

Tom
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Post by Tom »

dannysauer,

If you have a 20x4 VFD with a keypad input, then you must have the VK204-25. You can try the following modifications to change the unit to TTL communications.

TTL communications will be on the same lines as the RS-232 lines. TTL and RS-232 is almost the same except for they are different voltage levels. In RS-232 levels -12V indicates a high, and +12V indicates a low. TTL communications 5V is a high, and 0V is a low. The main reason why people use RS-232 communications is that you can communicate over long distances without any interference because of the high voltage swing. By doing the modifications, we are bypassing the RS-232 chip.

I hope this information helps. If you have anymore questions or concerns, please feel free to post them.

Best Regards,
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dannysauer
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Post by dannysauer »

Ok, so I cut the traces, and then bridge the two gaps in those two solder pads where it says "solder", and then I'll have TTL levels on the DB-9 where I would have originally had RS232 (ie, the receive and transmit are on the same pins as the RS232 Rx/Tx)?

Is it possible to power the display over the DB9 connector, too, so I can just have a single plug? It seems like PC serial ports have a +5V and ground pin already...

Miles
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Post by Miles »

Yes, the Rx and Tx pins will remain in the same orientation for TTL as they were for RS232. You can apply 5 Volts ONLY via the DB9. CAUTION: Voltage input via the DB9 is not regulated, therefore you must apply 5 Volts only. To apply voltage via the DB9 connector you must solder the "solder jump" located just below the DB9. Please let me know if a picture is required of the solder jump.
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dannysauer
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Post by dannysauer »

I'm pretty confident that the LM2937ET-5.0 I'm using will produce a stable 5V. It's supposed to be good for "in excess of" 500mA with just the on-die heatsink, thermally limited by the internal thermal shutdown, and I've got a pretty substantial heatsink on it, so it should be capable of handling the 550mA inrush with no problem, let alone the 25mA operational draw. It's odd that the data sheet says this would have a 290mA backlight, though - I thought VFDs weren't backlit? Is that just the power drawn by the display itself, forced into the closedt column on the data sheet, or was that number thrown in for some other reason? I'm designing for 315mA constant draw from this display, with a momentary peak of 550 - lemme know if that's a bad assumption. :)

Tom
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Post by Tom »

You will need the 550mA inrush current to drive the VFD. The VFD is not backlit though. The 290mA may be a typo on the website. I will get this straightened out. I apologize for any misconception this may have caused you. The operational current is approximately 70mA, but you should design for the momentary peak of 550mA.

If you have anymore questions or concerns, please feel free to post them.

Best Regards,

dannysauer
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Post by dannysauer »

Ok, I'm sure I can handle the momentary 550mA (could probably sustain 550, but I'd rather not push things), and honestly I'm fine with having a design anticipating about 300mA only having to deal with 70. That's better than the other way around! :)

Thanks much. Hopefully I'll have a PIC 16F688 talking to the display this evening.

dannysauer
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Post by dannysauer »

Doh! My VFD is an older revision of the board, and those solder pads aren't present. Do I have any options? I don't see a solder jum under the DB9, either - does that mean I'll have to feed 5V on the power thing like before? Is there an "upgrade" path for this board? :)

BTW, what's the 3-pin "CN2" on the front board for?
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Tom
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Post by Tom »

Unfortunately that revision is pretty old, so you'll need to do a lot more work on the display to have it communicate via TTL. It will require some cutting of traces and wiring to bypass the 232 chip. You will still need to have the 232 present for the contrast. I suggest you use a TTL to RS-232 converter. That would be the easiest alternative. If not, then I'll help you figure out the connections on the board.

The 3 pin "CN2" is for testing out the vfd. It is to check whether the display driver is working properly.

Best Regards,

dannysauer
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Post by dannysauer »

Well, heck. I've got a couple of max232s sitting on my desk too, but I've never used those, or really even looked at the datasheet. Is it as simple as just hooking it up inline, or do I have to hook a bunch of other stuff (which my local Radio Shack probably doesn't have) up too? Pointers to an example circuit would be helpful. :)

Tom
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Post by Tom »

They are really quite easy to hook up. All you would need to do is refer to the data sheet. You can refer to figure 5 on page 17 of the max232 datasheet shown at http://pdfserv.maxim-ic.com/en/ds/MAX220-MAX249.pdf

I hope this helps you out. If you have anymore questions or concerns, please feel free to post them.

Best Regards,

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