Micro­power

  

Volume 2 · Number 2 · April 1982

Page 32 of 37

We are now in a position to start piggybacking the RAM chips. Pin 4 of every socket should be bent out at right angles to the socket. The remaining pins should be soldered using the smallest bit available. As you complete each 4116, it is a good idea to put it back into the board and test it. (If you are soldering chips directly onto chips, you must take pin 4 of the top IC to +5 volts when testing. If the pin is allowed to float, neither IC will work correctly, and the memory test will fail.) When all eight chips have been piggybacked all the pn 4s should be commoned by connecting them to a wire link running along the bank. As a further check the top ICs can now be inserted and if the linking wire is pulled to +5 volts through a suitable resistor (I found that 330 ohms was O.K.) the board should function normally as a 48K board at this stage.

Now that we have the fourth bank of chips mounted successfully on top of one of the other banks, and the board is working normally, we can start looking for the extra logic signals that are needed. If you look at fig. 10 of the RAM-B instructions you will see SK1 where the DIL header plug (4K BLOCK DECODE) is inserted at the top left of the diagram. Pin 20 is labelled NAS1 SEL and goes direct to MEXT on line 11 of Nasbus. If you are working with Nascom 2 or Multiboard, this signal is not required, and pin 20 of SK1 provides an ideal point for the connection of the RAM BLK3 signal. Cut the MEXT line on the solder side of the board where it runs from the plated through hole opposite pin 11 of IC43 to the hole opposite C52. I strongly advise that you check that you have the correct line by using a multimeter or some other form of LOW VOLTAGE continuity tester before you do any cutting.

You will see from diagram 10 that RAM BLK0/1/2 signals go to pins 4/2/1 of IC35. We need to connect pin 20 of SK1 to pin 5 of IC35. This pin is connected to +5 volts by a wide track which is hidden by the socket, so the simplest way to make the connection is to remove the IC from the socket and bend pin 5 at right angles so that it remains clear of the socket when reinserted. A wire should be soldered on this pin and taken to intercept the line from pin 20 of SK1. Look at the area on the component side of the board between IC35, IC36, RP5 and IC42; there are two through plated holes in this area, and the one nearer to IC35 is on the MEXT line and connected to pin 20 of IC35. The wire from pin 5 of IC35 should be connected to this point. Once again, check carefully that you have the correct point.

The RAM BLK signal lines are pulled up to +5 volts by 1Kohm resistors R7, R8 and R9. RAM bLK3 must be pulled up by a similar resistor. A convenient place to do this is beside SK1, where there are two plated through holes next to the silkscreened numbers 7 and 9; the one near the 7 connects to pin 20 of SK1, and the one near the 9 is at +5 volts. The MEXT line is already pulled up to +5 volts by a 4K7, so to achieve a pull-up of 1 Kohm the resistor used should be 1.2 Kohms.

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