What is the Serial Voltage used on C/CON16 Units?

I have a customer that I have migrated from an older AccelePort C/CON-16 to a ConnectPort TS16 everything works except for one machine.

I can communicate to the CNC machine using the C/CON-16 device but when I try using the ConnectPort the CNC only get a small portion of the file. Communicating from the CNC back to the Computer works great.

I have searched but I am unable to find any documentation on the Serial Voltage used on the older C/CON hardware. I am thinking the C/CON device us pushing a higher voltage than the new ConnectPort.

This is all I can find on voltages
The important details of the RS232 signaling are:
Tx output voltage min is +/-5V
Rx input threshold voltage is +/3V

Thank you.

Any PDF files or other documentation?