Epson Multi-mode Data Controller MFJ-1278B - Manual (Page 353)

MFJ-1278B MULTI-MODE APPENDIX E CONVERTING TO TNC-2
3. Construct Header U28 by connecting pins 2, 5, 6, 12, 15 and 19 to ground pin 10.
Connect pins 9 and 16 to +5 Volts at pin 20. See Figure E-2 below.
Fig. E-2: Header U28
4. Remove U27 on the MFJ-1278B PC board and replace it with Header U27.
5.
Remove U28 on the MFJ-1278B PC board and replace it with Header U28.
6.
Set the terminal program parameters as specified by the NET/ROM documentation.
Upon power on the MFJ-1278B should signed on like a TNC 2 and it is ready to operate
NET/ROM or other TNC 2 firmwares.