29 Apr
2004
29 Apr
'04
11:03 a.m.
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