Digital (serial) data has a number of attributes including voltage, polarity, data rate. The KPro-III input uses 5V idle / 0V active voltages with a variable data rate. This is often referred to as TTL level serial.
A computer serial port uses the RS-232 standard, which uses -12V idle / +12V active voltages. Many serial devices output a non-standard serial which is TTL level voltages but with RS-232 polarity (0V idle / 5V active). These signals have the polarity inverted for the KPro-III digital input and must be converted.
PLX iMFD devices output 3.3V / 0V (which works).
Serial output from two PLX devices in daisy chain.
Innovate LC-1 and AEM 30-2310 output 0V / 5V, which will not work.
Serial from the Innovate LC-1. Note polarity and invalid serial frame.
A TTL to RS232 converter will change the voltage and polarity to 5V / 0V, which works. The KPro4 has an inverted digital inputs 'I-' for inverted data streams.
Serial from Innovate LC-1 after conversion.