Question about MAX232

Archive New Product Ideas
1

OK, I need to add a serial connection to a cable modem that I have.

I’ve noticed a couple of parts on the site that I think will suit my needs, but I have a question about some of them.

I know that I need to convert from TTL (0v for logical 0, and +5v for logical 1), to serial.

Now, this part:

http://www.sparkfun.com/commerce/produc … cts_id=316

is the MAX232 chip, which will do just that.

I also found it on a PCB:

http://www.sparkfun.com/commerce/produc … cts_id=449

Then, I also found this:

http://www.sparkfun.com/commerce/produc … cts_id=133

my question is, will the last product work in the same way as the first 2? (the first one i would have to finish myself, i know)

The reason i ask is because in the last item, the description says that basically all new computers use EIA-232 standard, which uses 0v and 5v, just like TTL. This is what confuses me, because wouldnt this render the first two products useless?!

Is there an error in the description, or am I just reading this wrong?

Any help is GREATLY appreciated, as this is my first attempt at doing anything with hardware (I have an Altera UP2 board, which has 2 FPGA’s on it, which I’ve never used before, and would like to start using, any help with what to do with these would be greatly appreciated also!)

Thanks much!

~Nate

2

I’m confused; I think the sparkfun web page must be wrong. As I understand it, TIA/EIA 232 is basically the same as RS-232x, it’s just that the standards body changed its name over the years. Perhaps EIA-232 modifies the spec so that the computer can be assumed to accept 0-5v levels, even though the “normal” levels are still +/- 9 or 12 volts? Normally, the range from -3v to +3v is a dead zone, and you can’t rely on a particular interpretation of a voltage in that range, but that dead range could be changed with a revision of the spec.

Apparently the standard is up to revision F. I mostly hear about revision C (RS-232C), and I don’t know what revisions D, E, and F brought.

3

So, any of those options should work then?

4

I think the last one is a little funky. It will probably “work”. The advantage is it consumes far less power as compared with the MAX232 (which consumes tens of mA).

5

Options 2 and 3 are the same circuit using transistors as level shifters. This will work with most newer PC serial ports and is really great for battery powered equipment. I did run into one situation where this approach would not work talking to a piece of test equipment that was expecting actual RS-232 levels.

Using a MAX232 will ensure that the proper voltage levels will be produced, but the circuit will require a little more power. I haven’t measured the actual power consumption, but I don’t think it is in the tens of milliamps. I know that the spec for the TI version of the MAX232 lists it at around 8 ma.

You have to decide on your project requirements, but the MAX232 approach is the most flexible as it will be able to communicate with any “RS-232” serial port. There are a number of chips in the MAX232 series including 3v versions and versions that do not require the external charge-pump capacitors.