> On one of my Futaba VFDs (inside a "Pole Display", just like the kind
> you see at WalMart - 20x2 in a rectangular box on top of a 1-1/2" pipe),
> has a _really_ simple RS-232 level shifter on a small PCB that sits on
> the back of the VFD itself... it's no more than two resistors and a
> transistor. The Futaba display wants TTL serial (+5V/GND), but the
Really dirty trick, used in a lot of commercial modems :
Feed the RS232 signal through a resistor in to a 74HC or 74HCT input. Use
the protection diodes of the chip to clamp the signal. Preferably add a
pull up/down resistor as well.
No, I don't like it either!
> Pole Display spec is (over a 6-pin DIN identical to what C= used for
> their IEC bus) RS-232 serial in and about 7-9VAC (which is rectified
That, actually, is the standard 6 pin DIN connector.
> and passed through a 7805 on that same little daughterboard).
>
> "Official" RS-232 is anywhere from +/-3V up to +/-15V (check the
> output of a laptop sometime - it's nowhere close to +/-15V). TTL
Doens't that depend on the Laptop. I have an idea (without proof, and the
service manual/schematics are upstairs) that the HP110 and Portable+ gave
out rather more than +/-5V on the RS232 port.
> never goes negative. Check to see that your "direct leg" isn't your
> ground. Those transistors and resistors could easily be your level
> shifters. Worst case, though, if you are only sending to the
> device and don't have to read from it, is to send it TTL levels
> (buffered of course, to protect your sending device), and it may
> well be able to receive. You'd need to know the character protocol
One other trap. Almost all RS232 receivers (including the 1-transistor
one) invert the signal -- a +ve voltage on the input gives a TTL 0 on the
output. So you may need to put an inverter between your TTL serial output
and the device.
Admission time : I have once used a 74LS04 as an 'RS232 driver'. It
didn't meet the spec at all, of course, but since I knew the
characteristics of the only port I was going to use the device with, it
worked. I also once used a 1489 (qwuad RS232 receiver -- converts RS232
to TTL levels) as a driver too -- A TTL signal will meet the input spec
of that device, and the output will also work into the input of another
1489. Meant I could use 1 chip from the junk box to link my TTL design to
an RS232 port.
No way would I do something like this in a device that was going out to
somebody else, or one which I was going to use on a variety of machines.
-tony
Received on Thu Apr 29 2004 - 18:03:22 BST
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