Prop to MAX232 into another MAX232

Zap-o

I was wondering if any one has used the propeller with a max232 ic and a male DB 9 connector. I have always used a max232 to receive data from a computer, but now I drew up a circuit that sends data through a max 232 (using the propeller) to a device that also uses a max232.Its not working correctly and wanted to know if anyone has a suggestion. Attached is the schematic of the max 232 chip that connects to the propeller.

Thanks

Mike Green

You cannot use a MAX232 unless you include some kind of 5V to 3.3V interface between the MAX232 and Propeller. A 2.2K resistor will usually work although there are other techniques as well. You can avoid that by using a MAX3232 instead which operates off 3.3V and interfaces directly with 3.3V logic.

The Propeller Professional Development Board (PPDB) has a MAX3232 and a DB9 connector. Have a look at the schematic and manual for it.

Zap-o

Thanks for the reply mike. I do have a 500 Ohm resistor in between the propeller and the max232. Ill put in a 2.2K and see what happens.

Mike Green

Other than the mismatch between the 5V and 3.3V logic levels, what you suggest should work and is used all the time.

Dr_Acula

Can you just clarify the pins on the D9 on the schematic? I see pins 3, 5 and 6.

For a D9 3 wire connection, use pins 2,3 and 5.

I remember a simple rule - male D9s output on pin 3. From that you can conclude that females output on pin 2. Pin 5 is always ground. Ignore the rest. Generally a PC has a male D9 (real D9 on the back, or a USB to D9 cable off ebay for $2). So this will go into a female D9 on pin 3. Which should then go into either pin 13 or pin 8 of the max232. Yours goes into pin 7 which is an output.

Unless you are outputting to another device?

I think you are very close. Maybe it is just the series resistors as mentioned above.

Also another rule of thumb - all RS232 signals are 'resting' at negative volts, eg -9V or -12V. But the TTL equivalent of 'resting' is 5V. (3V on the propeller). So you should be able to trace through a resting signal
+3V on the prop => -9V RS232 => wire => max232 at the other end => +5V => 1k resistor => 3V on a propeller.

See my link below - that takes you to a page with a schematic showing a max232 to implement two serial ports on a prop.

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www.smarthome.viviti.com/propeller

Leon

A MAX232 will probably work at 3.3V. I've used a MAX202 (5V) at 3.3V until I was able to get a MAX3232, before now.

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Leon Heller
Amateur radio callsign: G1HSM

John R.

You may need to have a "null modem" cable between the two devices, in other words, Pin 2 to 3 and 3 to 2 (among other cross connections). If you have a "straight through" cable, there is a good chance that you are connecting the two receive lines to each other and the two transmit lines to each other. This doesn't work so well.

I believe this is another way of saying what Dr_Acula is describing.

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John R.
Click here to see my Nomad Build Log

Zap-o

Dr_AculA

That rule of thumb works. Thank everyone for the help. So to conclude I put a 4K resistor in between the propeller and the max 232. And I had to re wire the pins as Dr_Acula mentioned.

Thanks all