Serial communication trouble when using J11

[plevrier]

Hi,

For an embedded application I am trying to use the P175 connected RS232 by using J11 onboard TTL level. It saves me an MAX232.

However, there is no communiction possible, until I open the USB RS232 port also. As soon as I close the USB-Serial port on my PC the communication stops after about 10 seconds on J11. Is this a known issue ? Is there a solution for this other than using the normal RS232 port ? eg removing some parts on-board ?

regards
Pascal

[Jan]

The J11 is connected the same way to the system like the internal MAX232 TTL side. Thus if standard RS232 communication works, the J11 must work as well.

Maybe your CPU is not able to overdrive the P175's internal 2k pull-up resistor on TX line or the High/Low levels are out of specs. Some MCU's have output pins with only 2 mA driving capability, this may be insufficient.