TNC1_01.TXT

Switch Selectable Firmware on the TAPR TNC-1
  By Lynn W. Taylor, WB6UUT  

Recently, I was faced with a dilemma.  New TNC firmware
written by someone here in California offered some nice features
not available with the standard TAPR firmware, while new software
products for the Mac and supporting an earlier project required the
standard fare.  Switching EPROMs (even with ZIF sockets) still
required removing the screws in the case and reinitializing.

Studying the TNC design itself provided the answer:  The TNC
presently "bank selects" two sections of NOVRAM by switching the
high-order address bit.  If the existing 2764's in the TNC are
replaced with 27128's would double the ROM capacity of the TNC. 
Simply adding the high-order address bit to the NOVRAM bank select
would then allow two versions of TNC firmware to be resident
simultaneously, each with it's own bank of NOVRAM for operational
parameters.

The modification can be performed simply as follows: 

1)      Remove U5, U9, U10, U11 and U12.  
2)      On the component side of the board, follow the trace from U5
pin 21 to the via near the zero in U10 on the silkscreen. 
(note: a "via" is a small pad used to pass signals from one
side of a board to another).  Verify this by measuring
continuity from U10 pin 26 to the via, and from U5 pin 21 to
the same via.  
3)      Cut the trace entering the via on the component side.  I also
recommend cutting the same trace where it connects to JP-8 to
eliminate the possibly of noise problems.  
4)      Add a wire from the via to pin 17 of U27 -- the NOVRAM "bank
select" pin.  
5)      If you intend to bank select all 4 EPROMs (not necessary in
my case), add a wire from U9 pin 26 to U10 pin 26.  

CONTINUED IN FILE TNC1_02.TXT