> The other thing I don't get is why use the optoisolator relay setup The article uses the optoisolators and relay drivers, not the relay itself (for the seria

> The other thing I don't get is why use the optoisolator relay setup The article uses the optoisolators and relay drivers, not the relay itself (for the serial data). And as shown in the article linked, it's not setup to be galvanically isolated. Given that the TTY has larger coils and motors, and might also be grounded to some other machinery (e.g. the mains protective earth), I can see why one would want to break the groundloop to the Arduino or serial/USB connector on a PC. The relay modules seem to have one control input wired: Vcc, In1, In2, GND. And a separate jumper for Vcc and Vcc-JD (as seen on a datasheet I found on the web for the 4ch version). http://www.sainsmart.com/arduino-compatibles-1/relay/4-channel-5v-relay-module-for-pic-arm-avr-dsp-arduino-msp430-ttl-logic.html http://www.sainsmart.com/arduino-compatibles-1/relay/4-chann... If you'd open the Vcc-JD-VCC jumper, have a 5V supply at JD-VCC and GND, supply 3.3V from your arduino to Vcc and connect In1/In2 to your GPIO/serial TX, then you could have your TTY and Arduino potentials completely separate.