All the multiplexers have Led lamps that signal activity on the inputs or outputs and other situations.
Nmea2Wifi KIT and TH - Green Led to indicate data being received at port P1; Red Led to indicate data being received at port P2; Blue Led (in the ESP8266 module) to indicate data being sent out on port P3
Nmea2Wifi SMD - Green Led to indicate data being received at port P1; Red Led to indicate data being received at port P2; Blue Led to indicate data being sent out on port P3 and Yellow Led to indicate data being received or being sent out by Wifi.
Nmea3Wifi and Nmea4Wifi - Inputs have Green Leds and outputs have Red Leds. There is also a Blue LED that blinks when data is being received by Wifi or is being sent out by Wifi. TH (Through Hole) models also have a Red LED which indicates that the unit is turned on. The Leds in the SMD (Surface Mounting Devices) models are very small but the are as bright as the 3mm Leds used in the TH models.
Here is a picture for the case of the Nmea3Wifi TH multiplexer
Power Up - On the Nmea3Wifi and Nmea4Wifi multiplexers, after power up, the Leds blink in succession from left to right (as defined in the previous picture) to indicate that the AP network was created successfully. If connection to an external network was set, Leds blink in succession from right to left when the connection is achieved.
Hard Reset - when the Hard Reset switch is pressed, all Leds turn on. The switch should not be released while the Leds are on. After 2 or 3 seconds the Leds turn off and the switch can be released for the Reset to be taken. Otherwise it will be ignored.
Hardware Testing - It is possible to test the serial output driver and the input optocouplers using a procedure described in the forum. The Leds will blink or not blink to indicate normal operation or a failure.