Bulletins from the Pacific Packet Radio Society - page 072


Date: May 18th, 1982
To: Hank, KA6M
From: Steve Grossg WB2GDQ
Re: VADCG TNC Memory
CC: Paul Rinaldo for AMRAD Bulletin

Here is the summary of memory modifications which you requested me to document:

I have wired up my board for 5 volt only EPROMS, but I am having trouble finding 1 6K Rams in a 4K by 4 arrangement (which can be placed where the 21 14's are currently). This can be done by replacing the 1 8 pin sockets with 20 pin sockets as all the wiring stays the same except for the two new pins and the old VCC pin which becomes an address line. The only versions which I have found so far are the 70ns (and faster) Inmos chips, although the 2K by 8 variety seem to be quite cheap and in large supply. Unfortunately, there are only 4 - 24pin sockets on the board~ so modifying it to handle that configuration by adding additional sockets would be significantly more difficult.

So, for the moment, I think I'll leave the address decoding as is and use 2716's as though they were 1K by 8 roms. At least that way I can use the 5V only chips and eliminate the -5V supply from the board. The modification to do so is very simple. The +1 2 volt and -5volt lines on the 2716's are disconnected from their respective power supplies (just where they connect to the supply rail) and are replaced as follows:

pin old 2758 2716 2732
19 +1 2v gnd A10 A 10
21 -5v Vcc Vcc All

For the 2716 and 2732 mods, either an additional 74LS138 must be added or ideally, 2732's and 4Kx4 static rams can use the same 74LS138 and just change the following inputs:

pin name old new
1 AO 8085-22 8085-25
2 A 1 8085-23 8085-26
3 A2 8085-24 8085-27

Note that the 8085 pins are mislabeled on the VADCG TNC schematic and the pins should be as follows:

8o85-21 A8
8085-22 A9
8o85-23 A10
8085-24 All
8085-25 A12
8085-26 A13
8085-27 A14
8085-28 A15

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