Bulletins from the Pacific Packet Radio Society - page 075

test of the TNC, and usually will fit in one 2708. Test your completed TNC to make sure it functions properly. Refer to Fig. 1, 2 and 4 while doing this mod. I have laid the mod out in steps to make it easier to follow and check off. Cuts on the board are shown as a small X thru a trace.

1. Cut traces between U8 pins 7,9,10,11 and U19 to U26 pin 8. The cuts are done by U8 with pins 7, 9, and 10 cut on the top (Fig. 1), while the pin 11 trace is out on the bottom of the board (Fig. 2).

2. Cut the trace at U8, pin 5 to ground (Fig. 2). Hook U8, pin 5 to U1 (8085) pin 28 (Al5).

3. Drill two holes as shown in Fig. 1. These holes facilitate the wiring of U27. Be careful not to drill thru any traces.

4. Add IC-27 (74LS138, or 8205) to shown on Figure 1. It can be board if necessary, or use tape. Put a 16 pin solder-tail to allow replacement in case it and wire the pins as follows: the board as glued to the double-sided socket to it should fail,

A. U27 pin 1 (A0) to U1 pin 23 (Al0).
B. U27 pin 2 (Al) to U1 pin 24 (All).
C. U27 pin 3 (A2) to U1 pin 25 (Al2).
D. U27 pin 4 (El) to U1 pin 34 (IO/M*).
E. U27 pin 5 (E2) to U7 pin 11 (RD/WR)
F. U27 pin 6 (E3) to U1 pin 28 (Al5).
G. U27 pin 8 (GND) to ground bus as shown.
H. U27 pin 12 (03) to U25+26 pin 8 (CS*) at cut
I. U27 pin 13 (02) to U23+24 pin 8 (CS*) at cut
J. U27 pin 14 (01) to U21+22 pin 8 (CS*) at cut
K. U27 pin 15 (00) to U19+20 pin 8 (CS*) at cut
L. U27 pin 16 (+5V) to +5 volt bus as shown.

Now that U8 is only used for EPROM's while U2 7 is us ed for RAM, U8 doesn't need to be enabled selectively with WR9 but just with RD from U1 U27 needs to be enabled with either RD or WR from U1 These changes are done by rewiring U7, the 74LSOO as follows:

A. Cut the trace between pins 3 and 4 of U7 on the bottom of the board (Fig. 2. ) This trace is thick; so check with a meter to be sure it is totally out.

B. Cut the trace between pins 6 and 10 of U7, located on the bottom of the board (Fig. 2) .

C. Cut the trace between pins 14 and 13 of U7, also on the bottom of the board. This is

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