Bulletins from the Pacific Packet Radio Society - page 074

From: Paul
Subj: VADCG TNC Memory Expansion

Thanks for the info from Steve Gross, WB2GDQ.. A s coincidence has i t , Terry Fox, WB4JFI wasn't able to go to Dayton because of work commitments. Instead, he stayed home and worked up a mod to the VADCG TNC board. Here is an article, written by Terry, for the June AMRAD Newsletter which is ready to go to press now. (You'll have to wait until you get the newsletter for the Figs.)

VADCG TNC Modifications Terry Fox, WB4JFI

I have found that we are rapidly approaching the limits of memory on the Vancouver Amateur Digital Communications Group (VADCG) Terminal Node Controller (TNC) board. It is just barely able to handle the link level software in the four 2708 EPROMs used on the board. This means that any network-level software must be done external to the TNC board. One approach is to come up with a new TNC board that has more memory on it, both RAM and EPROM. The other option is to expand the capabilities of the VADCG TNC board. I should point out at the outset that this modification is a kludge, but at least it works! Only one IC is added to the board, and it is put on the board dead-bug fashion (upside-down).

This modification is done in two parts, first the RAM on the board is moved from its present location (1000-lFFF hex) to a new location to allow more EPROM memory. The new 1ocationis 8000-8FFF hex, which should allow up to 32k of EPROM memory. This location also simplifies the hardware modifications required. The second half of the modification changes the EPROM sockets to allow the use of either 2716s or 2732s.

Relocating RAM to 8000 Hex

This part of the modification requires the addition of another IC, called U27, which can be either a 74LS1 38, or an 8205. U27 becomes the RAM address decoder, while leaving U8 the EPROM address decoder. I should point out that the schematic that I received with my VADCG TNC board has some minor errors. One of these is that address lines A8 thru A15 from the 8085 (IC-1) are backwards. A15 is pin 28 of the 8085, A14 is pin 27, etc... down to A8 being pin 21. Using the proper pins is important to this modification, so be careful of this error.

When this portion of the mod is completed, the TNC should still operate using 2708s for the EPROMS. I recommend that the board be completely assembled and tested before doing any modifications. Have someone burn a couple 2708s with repeater software for your call in them, one version for RAM at 1000 hex, and the other version for RAM at 8000 hex. Even if you don't use your board as a digital repeater, it is a good

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