Hi All,
I am having an issue with the TTL output from a MAX242 (18 pin version) despite the enable pin being in a low state. I have confirmed with my oscilloscope that there are RS232 signals on the RS232 input (pin 14), but I do not see any output on the TTL side (Pin 13). The data sheet shows that the enable pin is active low which I have confirmed with a DMM that it is. Shutdown pin is floating at 0V.
I have the Arduino setup to output "RS232 comms active" each time the reset button is pressed for testing. When I do this, I receive the data and that text shows on in the tera term terminal. When I press characters on the keyboard, I can see the RS232 data present at Pin 14 but nothing on Pin 13. I am placing the oscilloscope probe directly on the pin of the IC. I have also confirmed I have the +/- 10V on pins 3 and 7 so I know the doubler and inverter are working properly.
I originally breadboarded this circuit and had the same issue. I have since built a prototype board in case there was some issue with the breadboard setup. Please see attached pictures.
I have also tried 2 other new ICs of same part number with the same results. Also, I have tried this with Pin 1 floating and at 5V, in both cases the voltage on Pin 13 is 0V and there is no data present when tested. When Pin 1 is low the voltage on Pin 13 is about 3V which I assume is a good sign. I just don't see any data on it, I know I have the voltage per division down low enough to be able to see it on the scope as when I test on the TTL Tx pin (pin 12) I can see data going out to the laptop.
Any help is greatly appreciated.
Thanks!
Joe
