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Upgrading your C16 to 64Kb

The virtual Plus 4

How many times have you heard from PLUS/4 fans, “Your C16 is only good as a PLUS / 4 parts depot”? How many times have you been offerred money for your C 16 by these same TED-chip-seeking adepts? Disregard temptation because now your C16 can be turned into the pseudo-equivalent of a PLUS/4 with the ability to run a great deal ofPLUS/4 software, You see, the C16 and PLUS /4 belong to the “264” series of Commodore machines. The C16 is actually a PLUS /4 with a few chips missing, notably the 6551 UART (and userport) and a full 64 KB of memory. In this article, we will be upgrading a C16 to a full 64 KB, allowing us to run many PLUS/4 or “generic 264” software (minus userport-terminal programs and some assorted misc. software).


Before starting read this carefully and check out the schematlcs in your CI6 manual. You should be familiar with soldering. I have reconstructed this, it is about 10 years since I have done this. I can give you no warranty, do it on your own risk! Neither myself nor the staff of the dlsC=overy journal is responsible for any use or misuse of informatlon presented in this article.

  1. Buy two 64×4 bit dynamic RAM chips llke “TMS 4464” or “41464”.  They should cost less than US$10 together.
  2. Remove your two old ”TMS 4416″ RAM chips from your C16. They are labelled U5 and U6. You might destroy them, but be careful to not destroy something else! I have used scissors to cut the pins and then I have desoldered the pins.
  3. Now solder two 18 pin sockets in where the old RAM chips have been. Again do it carefully to avoid destruction! Check out where pin 1 is. Then plug In the new RAM chips.
  4. Check everything again, then switch on your C16. It should still show “12KB free”.
  5. Adress lines A0 to A13 are connected to the multiplexers U7 and U8.  You need to connect A14 and A 15 to them instead of +5V at U7 pin 2 and U8 pin 14 (but the +5V connections at pin 16 must be left).  For example get them from the CPU (U2) pin 21 and 22. First scratch the +5V connections, U8 pin 14 is connceted on the lower side only and should be easy to disconnect. U7 pin 2 is more tricky.  Disconnect the line between pin 2 and pin 16 at pin 16 on the upper side (U7). Pin 16 still gets +5V from the lower side.
  6. Now you have two choices:
    1. To always have 64KB:
      Connect U7 pin 2 with U2 pin 21 (A14).
      Connect U8 pin 14 with U2 pin 22 (A15).
    2. To choose between 16KB and 64KB:
      You need a double switch (or whatever this is called. it has 6 pins). Connect U7 pin 2 and U8 pin 14 to the two middle pins of the switch. Connect U2 pin 21 and 22 to the two upper pins of the switch. Connect +5V or Ground to the two lower pins of the switch. With that you can chosse where your 16KB area is located in your 64KB area. I have used FB 13 to get Ground and FB 14 to get +5V.
  7. Check everything again carefully. Then switch on your C16. It should show “60KB free” if the switch is in right position.
  8. It is a good idea to replace the 7805 (labelled VR1) with a 78S05. because the 7SS05 switches itself off. if it gets too hot.

Remember to keep the connections short.

Categorized as: How to...

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