I have an UPS I want to control through its DB9 RS232 port. (I know its pinout is pin 5 for GND, 2 for Tx and 3 for its Rx)
I already managed to communicate with the UPS from a PC running linux and screen:
screen /dev/sttyUSB0 2400 Q1
For instance !
Sending a "Q1" retrieves data from the UPS
Sending a "S1R000" will stop it in 1 minute.
Sending a "C" would resume to normal mode,(power ON again)
I've been doing so using a general purpose USB to RS232/RS485/TTL converter that works ok.
But I'm aware that normal RS232 can go from something like -12 to +12V, some UART are on 5.0V (UNOs) and some others on 3.3V (ESP8266).
I'd like to remove this converter and use directly the Arduino's or ESP8266's UART, so I need to know the voltage level of that UPS's RS232 port.
I've been checking with a voltmeter between pin 5 and 3 then between pin 5 and 2 of the UPS but can't find any voltage !
Would you have any advice to get that voltage ?