[SOLVED] MAX3221 and UBS converter crashes terminal software

[passpics]

Hi, I'm new to this forum, and hope you can help with a bit of a head-scratcher I'm having:

I have a custom PCB based around a PIC 18F, and using a MAX3221 to give me simple 3 wire RS232 comms (9600 baud). I've got this connected to a PC via an FTDI based USB <-> RS232 cable. This is a cable I've used for several years, and it works fine talking to other RS232 devices (I just tested it again talking to a MikroElectronica board).

However, here's the strange bit: When I use this cable connected to my PCB, the moment I transmit anything from the PIC to the PC, it crashes my terminal software on the PC! I've tried 3 different terminal apps, all do the same. Huh? Also, it seems like once I do this, my USB cable goes all weird and won't work until I re-connect it.

If I stick a scope probe on the Tx output from the MAX, I see +/-~6v (12v p-p) bursts of data at the correct times.

It's such a simple little circuit, I can't think what's going on. I've double checked, and all pin wiring seem to be correct. Here's the schematic of my PCB side of things:



Anyone have any brilliant ideas? Or see anything stupid I've done? Many thanks.

 

[jayanth.devarayanadurga]

Post code. Maybe data is sent continuously. Does the voltage levels of your micro and MAX3221 match?

 

[passpics]

Hi Jayanth. Code is somewhat large. I've actually had this running on the Mikro Electronica dev board, so I know the code is fine. Plus I've got a scope probe on the Tx output and just see short bursts of data as expected. Line holds at -6v the rest of the time.

 

[passpics]

FYI: Solved this problem:

1. I had my MCU running at the wrong oscillator speed (!), which meant the RS232 baud rate was much too low. Seems like my USB to RS232 converter didn't like this situation at all. Can't think why, but once I set the correct speed in software (and hence the baud rate was calculated correctly), all worked sweet.
2. When powering up my board there would be a spike on the RS232 line. I added a pull-up resistor to the MCU side of the MAX chip, which solved that problem.