Why is isolation needed for my RS232 connection?
The NMEA 0183 specification, from version 2 and higher, requires devices to be isolated on their input lines. The OPTO-4 allows an RS232 input connection to meet these NMEA 0183 Listener specifications.
Without this protection you risk ground loop damage to the RS232 device and/or the NMEA Talker. This might result in the need to replace expensive equipment.