LED displays (TIL305, TIL308, etc.)
ethan.dicks at usap.gov
Tue Sep 30 07:54:08 CDT 2008
On Tue, Sep 30, 2008 at 12:34:03AM -0400, Roy J. Tellason wrote:
> > > No 7446/7 chips on hand to drive them with, though. Or any of the
> > > 4000-series CMOS either. :-(
> > I have one or two of a few different types, but no loose 9368s (only
> > installed in boards already).
> I'm not familiar with that one, offhand (and don't have it in my big generic
> numbers chart).
I can't look it up to verify at the moment (no satellite) in case I have
some digits swapped, but the part I'm thinking of was used in the RCA
MicroTutor II (the later 1802-based one, not the earlier 1801-based one),
a hand-held 1802 device that generates a lot of discussion on the cosmacelf
Yahoo Group, and the Quest SuperElf, among other places. The part is, IIRC,
a Fairchild part that does the same sort of decoding and segment driving
as a 7447. I couldn't describe any differences off the top of my head
though (except a different pinout).
> > One thought I had for trying to use a reduced-pin-count MCU-based numeric
> > display was a 7447-type chip on 4-bits of an I/O port, and a 74145-type
> > BSD decoder on the other half of the port...
> Sounds to me like it oughta work. I have some nontrivial discussion of
> driving those devices in the _8085 Cookbook_, but it's been a rather long
> time since I read it. I do recall them talking about how important it was to
> have some way of not pushing too much current through multiplexed displays
> when things weren't running yet, or in case of program bugs, etc.
Yes, if you are depending on the multiplexing to limit your current. The
trick there is that a steadily-lit red LED might be able to take up to
20mA with no problems (10mA should light it, ISTR), so in that arrangement,
you select an input voltage, calculate the voltage drop across the LED,
then use Ohm's Law to pick the value for your current limiting resistor
(something like 220 Ohms for driving a bog-standard red LED from +5V).
The trick there is "steadily-lit" - if you are strobing an array of
individual LEDs, you can frequently get away with no current-limiting
resistor as long as you don't stop strobing and accidentially drive an
LED junction on "full". One advantage of this technique is that you can
up the momentary brightness compared with a non-multiplexed bank of LEDs,
helping to make up for the fact that you _aren't_ lighting each lit
junction all the time. This trick is apparently very helpful with IR
LEDs in particular, since a remote control is always sending pulses,
not lighting up and staying lit. You can pump a lot of instantaneous
current through the output LED, as long as you don't leave it on too
> > Just checking now, it looks like the CD4511 won't render any digits if you
> > give it an input over "9"
> I haven't looked at that aspect of that chip.
> > (the 7447 has incomplete decoding internally, so will light various odd
> > segments if you don't stick to 0-9).
> With blank for F IIRC, and I always wondered why they did it that way.
Thank you for mentioning this... I just read over an ancient TI datasheet
for the 7446-7449 family of decoder/drivers and there it is. I hadn't
really stared at that end of the decoded pattern to see that "F" renders
as blank. That could end up being quite handy.
> > With a CD4511, then (and a 74145 or CMOS equiv), one could hook up to 10
> > digits and just write "F"s to any positions you wanted to blank.
> I don't know what the heck I'd do with ten digits worth of that stuff
> though. :-)
I have several ideas, one of which is finishing the revival of a rackmount
GPS frontpanel (a Datum 9390). I was way too late to save the guts of it
when it was scrapped last year, but they let me have the 19" plate with
an 11 button input panel, an Optrex 4x40 LCD, and a 9-digit "clock"
(hours/minutes/seconds + day-of-year). It has some oversized HP 7-segment
LEDs mounted on a small board with a 20-pin DIP socket - I could pull the
digits from the board and wire them up (at the same visual spacing) any
way I like, but _if_ I can come up with a circuit that will drive nine
7-segment displays with the A-G segments in parallel and individual
common lines, I could give this frontpanel the appearance it had when
it was a working GPS. More modern versions have either a backlit LCD
or a VFD - no glowing red digits.
> I seem to recall Radio Shack selling a couple of chips for that purpose,
> 75491 and 75492.
I'm unfamiliar with those. I should look them up.
> The former had two sets of outputs but fairly limited drive
> capabilities, while the latter was open-collector but could drive a good bit
> more if I'm remembering right.
The 7446-7449 family has a similar spread of features. The 7448, IIRC,
can only sink 6mA and is designed to be externally buffered.
> They didn't seem to be too friendly to
> common-anode displays, which is what I was wanting to use at the time.
Ah... that is one of the reasons why TI made a spread of parts with
different drive capacities and other features.
> > They do make things easy, though, as long as they provide enough digits (the
> > DL/SA scoreboard has 16 total digits and a pair of ICM7218D to drive them).
> I'm going to have to see if I can remember to snag a datasheet on those at
> some point.
I think I got mine from the Dragon's Lair Project webpage years ago. I
snarfed some code from Daphne (the laserdisc game emulator) and bodged
it up to let me do a variety of things from the command line (set individual
digits, cycle through various patterns, and to even use the two 6-digit
score lines as YYMMDD date and HHMMSS time). The code to set an individual
digit is not particularly complex.
> I seem to recall Osborne using one of those, might've been the 6821, and I
> don't think it took too much in the way of glue logic to make it work on the
> z80 bus.
Nor the 6502 or pretty much any 8-bitter. The only thing that gets
sticky is when you have to fiddle between /WR + /RD lines vs a single
/WR-RD line ("motorola" vs "intel" bus standard as frequently described
in docs on textual LCDs and VFDs).
> > .... I did take a pair of raised-segment LEDs...
> I saw the picture of those but don't recall ever actually seeing those
I got mine in the late 1970s in a $2 multi-pack at Radio Shack. I don't
have as many as I used to due to youthful exhuberance and destructive
testing (hint: they can't take a 9V supply without a resistor ;-)
> > Oh, yeah. So are small textual and graphical LCD and VFD displays, but
> > there's a real appeal, to me at least, of the soft glow of red LEDs. Not
> > quite as cool as Nixies, but those evoke memories of a different era.
> Yep. I wouldn't mind getting some nixies to play with too, but those are
> getting way too complicated too. And are the driver chips for them available
> at all any more? What was it, the 7441?
The 7441 sounds right. I think someone rebadged a bunch of 7441s as 74141s
(or something similar), but they aren't particularly cheap. I think I have
one 7441 in a drawer.
> I have a few LCDs I've been meaning to play with, and some VFDs out of
> scrapped VCRs, most of which are too application-specific for me to want to
> do anything much with them but one or two of the more recently-acquired ones
> might be nice to do something with.
Many of those appliance displays are unformatted - the controller/driver
is embedded on the main board of the device. The ones I've played with
have an HD44780 or some graphical controller (depending on the display
type), and that takes care of strobing, driving voltages, fonts, etc.
You just worry about register-level formatting of what you want to put
on the display.
I don't have any experience driving "naked" VFDs, but I have a few
panels and wouldn't mind taking a stab at it someday.
> Tony mentioned some trick in the past
> week or two for a similar type of device, though I think that was only a
> single indicator, as far as making it work, and I was wondering how that
> might apply to a whole display...
Much in the same way as things that kicked off this thread to begin
with - you have to have some part (dedicated or a custom-programmed MCU
for example) that strobes through the various display elements (pixels/segments)
and that you can address externally to end up with the desired bits lit and
unlit. I have a few naked LCDs that I'd love to play with, but with
all the formatted LCDs that are out there, I've never gotten into
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