really simple RS232-TTL converter... does it work?

Pure TTL out usually works, and just a series resistor for the input direction, to limit the current. Data isn't inverted, of course.

John

Yeah, I've heard that. But the serial protocol is done in hardware on both ends (my computer, the system-on-a-chip in the router), so I couldn't use a non-inverting method. This trick with the MOSFETs does seem pretty good to me...

Unless you are driving a long line, I would avoid any 232 chip with a charge pump. They add jitter to the signal. [I've designed some of the Maxim parts, and trust me, it is something you check in the lab since the test engineer needs to know the limits of the jitter. I don't know if any datasheets show this.]

The typical 232 receiver is just an inverter, a resistor divider, and a diode clamp. The -3V trip points doesn't have to be reached as the trip point of the inverter is above ground.

Wow, you designed those? Cool! :-)

Thanks, that's very good to know. So if I have this right, most computers just use a circuit like this as an RS232 receiver?

RS232 LEVEL INPUT *------------+ | >

If so, then I can certainly see how 0V/+12V works as "RS232" signaling. Why does RS232 use such goofy logic levels anyway?

Dan

Naaah!

More like....

RS232 LEVEL INPUT *------------+ | > 'HCT Inverter < R1 TTL > LEVEL | |\\ OUTPUT +-----------------| |O-----* |/ The MC1489 (my design) has a (floating-tub) input resistor into the base of an NPN transistor, with a large-value feedback resistor from output to provide a modicum of hysteresis.

...Jim Thompson

Well, that is indeed pretty rudimentary :-) Can HCT logic handle the

Cool! I looked up the datasheet, and I see that the input level threshold for your MC1489 is 1.4V going up, and 1.0V going down. Interesting. And I guess that means that TTL levels are safe to use with it.

Thanks for all the help! I guess I'll just go with HCT inverters and be done with it! Dan

Using a P-channel MOSFET, you get a logic inversion, and the Tx data output swings negative. The negative supply may be taken from RTS, DTR, or even the other data line:

I don't anything anymore even puts out ±12V.

To be totally Kosher I'd add Ge diodes from the inverter input to each rail to keep current out of the ESD diodes on the chip.

The "standard" that never was ;-)

Yep.

That'll work just fine.

...Jim Thompson

Hmmm... I thought that RTS is held at +12V when not in use. If not, I don't understand how the circuit works that I linked to, since it uses N-channel MOSFETs.

Now I'm really confused. I just measured the serial port on this computer and RTS is definitely -12V. So how was that circuit that I linked to supposed to work???

Try enabling hardware handshaking in your terminal emulation software. Both RTS and DTR should then be at +12V.

Aha, yes, I see that those signals are supposed to be "asserted / logical zero / +12 V" when hardware handshaking is in use. So hardware handshaking would have to be enabled (though not actually doing anything) in order to make that circuit work.

Whereas your circuit would only work with hardware handshaking DISABLED, because it depends on having access to a negative voltage from the RTS.

Dan

If you want a negative voltage, you have to program one of the handshakes to be negative, or use the other data line.

You're getting the idea, but that is not quite right. There are 3 signals you can get the negative voltage from. Only one of them needs to be negative. The others can be positive e.g. for supplying power to a microcontroller. I usually derive a negative voltage from the computer's transmit data output, and I draw a positive supply from both RTS and DTR.

Gotcha. So you enable hardware handshaking, thereby driving RTS and DTR positive, and draw positive supply from it? How much current can they source?

One thing I still don't get: since RS232 is a full-duplex protocol, how can you derive a negative voltage from TX? When the computer isn't sending data, I understand that TX will idle at -12V, but isn't there a possibility that it will be switching between +12 and -12V rapidly when you're trying to use it as your negative supply?

Dan

Yup. In fact, 0 and 5 has worked fine on every computer I've ever used, including serial ports built in on motherboards, add-on PCI cards, laptops, and USB to RS232 converters. It's very convenient. It didn't work on a Palm handheld once; that required a proper negative voltage.

I suppose it depends what you want it for. Obviously you don't want to send something like that to a customer, but for you own use it will very likely be adequate.

Actually, the other circuit is closer to what was used. Just going to the inverter gate with a resistor would cause the impedance to be very high as the input went between ground and the positive supply rail.

The divider in the Maxim designs was needed to keep the gate voltage within limits. Actually, TI started to use those cheapie receivers, and we just copied them. The parts with automatic shutdown have an extra receiver that can sense if the input is at ground. The auto shutdown plus was a bit different, and I think really only done to prevent someone else from doing the design, i.e. create some prior art.

If you sweep the input of the 232 chip, you should be able to find the kink in the impedance when you go beneath ground

Very good to know that 0/5V output works with most computers. I really like the scheme that Andrew Holme has come up with, since it still requires only a MOSFET and a resistor to output +5V and -12V logic levels, which should really, truly work with any RS-232 device.

I think I'll give the HCT inverters a try first, since it's so ridiculously simple, and I imagine the worst I could do is blow up the inverter being used for the input.

Dan

Isn't the input impedance of a CMOS gate *supposed* to be extremely high?

Doesn't this simple voltage divider increase the quiescent current drawn by the device significantly? Is that why the shutdown mode is included?

That's the point. If you had a resistor in series with the gate, looking into the "port", you would see a high impedance. Once you hit the protection diodes, you would see roughly the series resistor.

The current in the resistor comes from the transmitter, i.e. the "other guy."

The shutdown mode first detects that nothing is connected to the receivers. If there is nothing connected to the port, then the on-board charge pumps (to create the voltages needed for transmitting) are just a waste of power, so turn them off.

I think Dallas (prior to the Maxim buyout) did a chip that stole power from the "other guy's" transmitter. Mice did this all the time. The mice that connected to serial ports did this. Generally they would set two transmitters high and one transmitter low. The two high transmitters would be diode connected, then used to supply the plus rail to the mouse. The transmitter that was low would supply the negative rail to the mouse. There was never a standard as to how much juice you could steal out of the 232 transmitters, so mice were always operating in a dicey mode. Needless to say, good riddance to serial mice.