MAX232 problems

[Art]

Old thread I know, but some of the posts mentioned are probably going to be mine,
but when I say MAX232, I actually mean Maxim/Dallas MAX232, which for hobbyists,
are, or at least were, the same price as Pics... zero dollars as free samples from the respective companies.

[E Kizer]

Nice to hear from you Art.

I just wanted to make a small comment.

The RS232 inputs of the MAX232 (pins 8 & 13) are pulled low internally with 5k ohm resistors. In fact any and all chips that have a RS232 input will have a pull down resistor ranging from 3k to 7k in accordance with the RS232 specification on their RS232 inputs.

What I was referring to was the TTL (CMOS) inputs for the RS232 driver. In every drawing on this and other forums the RS232 transceiver chip is drawn with only one input going somewhere, the other is left floating ( note; so using a MCU's weak pull up feature wouldn't work since there is no connection made).

There was a post of people experiencing problems with overheating of their chips. In almost all cases of overheating of any type of TTL/CMOS be it transceiver, Gate, Counter, etc it's because an input was left floating.

The Maxim MAX232 has internal pull UP (400k ohms) resistors on the driver inputs pins 10 & 11. It should be noted that there are many many variations of the RS232 transceiver chip. And not everyone uses the MAX232. Some users might prefer a chip that uses a smaller capacitor for the charge pump. Some just like a cheaper chip.

Personally I use the (cheaper) Texas Instrument TRS202. It has all the same features as the MAX232 except it doesn't have internal pull up resistors on pins 10 & 11. From the TI TRS202 spec sheet; "8.3.2, RS232 Driver, The driver inputs do not have internal pull up resistors. Do not float the driver inputs."

So you are correct in saying the Maxim/TI MAX232 doesn't need external Pull up/down resistors on the driver input. But not every RS232 transceiver chip offers that feature. Yes, read the technical documents. When it comes to working with RS232 transceiver chips research if the TTL inputs have internal pull ups. But don't assume they do.

I leave it in you capable hands to help the users out there that are experiencing overheating with their IC's. There are way too many variations of RS232 transceivers for me to keep up with. Personally I still follow the TTL/CMOS rule that every input must go somewhere ie hi/lo.

Thanks for your comments.

ed

[Art]

To be honest, I’ve never read such a post complaining about heat from any of them, but at the same time, am not here a whole lot these days.

If a chip was overheating while still functioning, oscillation from a floating input is the first thing I’d suspect,
but in modern times, anything other than stock logic is becoming more & more likely to include diode clamping, ESD/transient protection, etc.
Stock logic has become a thing of the past for most people other than me, and your typical PGA that replaces them will usually have internal pull-ups as well.

In modern times, I’d certainly replace rule number one with “Study the data sheet”.

[Dave]

If the parts you are using are original MAX232's then first I would look for the capacitor polarity's being correct. The original MAX232 devices required a 1 uF. cap for each of the up and down converters.