MAX232 problems

[E Kizer]

Occasionally I will search the forum for serial routine ideas. These searches always bring up old posts from people who have overheating issues with their RS232 interface IC’s. In every post I have seen here, that included a schematic, the schematic is wired incorrectly. Instead of posting how to correctly wire an interface chip, the threads sometime gravitate to discussions on fake chips from China. Though fake chips are a problem, the fix is to observe proper wiring of TTL devices.

TTL wiring rule number one;

All inputs MUST go somewhere.

Leaving an input floating can cause wild oscillations, overheating issues and massive current draw issues. The MAX232 is a two channel device with two TTL inputs. Every schematic posted in this forum shows only one input tied to something. The other is left floating. This is wrong. Tie that other, unused, input to either ground or 5 volts.

Another point of confusion I see is that “MAX232” is taken as literally, when it is really being used generically . When shopping for a new IC, it is better to search for a “RS232 interface IC”. A Maxim Integrated MAX232CPE from Mouser.com cost $3.20. I prefer to use a Texas Instruments TRS202ECN from Mouser.com which costs $1.53. I get the same performance at half the price. There’s no need to turn to China for affordable interface ICs.

So if you happen across a post that includes a schematic that shows an RS232 interface chip that is wired incorrectly, an input left floating, reply to that post to do something with that unused input. Doing that will save someone hours of trouble-shooting time.

[sayzer]

What is it that you did not understand on this circuit?

[Demon]

I always check the datasheet and check at a sample circuit there. Mainly to make sure what to use as pull-down/up resistor values (some people approximate in their circuits).

I know you can get away with approximations in resistors and capacitors in a LOT of cases. I just prefer to err on the side of caution, usually cause I end up on the side of user-error in many cases regardless.

Robert


EDIT: Take the MAX232, I'll even try to find the datasheet for that given manufacturer, just in case.

I know many chips "might" follow industry standards for a given chip, like a MAX232 (just an example), but again, I wouldn't trust a circuit I invested a gazillion hours putting together, only to have it "oscillate wildly" cause one manufacturer decided to be the lone ranger.

I'm not going to put anything in production, sadly, but I still want to be able to trust my crap when I do get it working, one day, eventually...

[richard]

TTL wiring rule number one;

All inputs should go somewhere.




CMOS wiring rule number one;

All inputs must go somewhere.


The problem with sayzer's circuit is that it will not work for max232epe chips , they will get hot real hot

the switching caps need to be ten times smaller 0.1uF

http://www.picbasic.co.uk/forum/showthread.php?t=19548

[sayzer]

TTL wiring rule number one;
....

The problem with sayzer's circuit is that it will not work for max232epe chips , they will get hot real hot

the switching caps need to be ten times smaller 0.1uF

http://www.picbasic.co.uk/forum/showthread.php?t=19548

Using ST232 and MAX232, I use 1uf elect type for many years now. Never had a heat problem.
For 232epe, some additional care may be required as Richard mentioned.

[Art]

Did you actually bother to look at a data sheet for any of the devices you are talking about?

The inputs of all MAX232 devices (at least authentic ones), are internally pulled low,
so that when left floating, their respective output is high, which is the serial idle state.
The RS232 inputs can also be left disconnected, and live inputs can be plugged in before the chip is powered.

Not bad what two minutes of research can uncover.

Similarly with Pics, you can turn on weak internal pull-ups and leave their inputs floating as well.