Assembling the Handy Board, Part 2

This is the second step in the Handy Board assembly. In this assembly sequence, the 32K static memory is installed and tested.

For this assembly sequence, collect the following parts:

• Capacitors:
  C4 (4.7 µF, labelled ``4µ7'')
  C5 (0.1 µF, labelled ``104'')
  

• Integrated Circuits:
  U2 (62256LP-120, 32K static RAM)
  U3 (74HC373, octal transparent latch)
  U7 (74HC132, quad schmitt NAND gates)
  U14 (LM2931Z-5.0, +5v regulator)
  

• Connectors and Sockets:
  DIP3 (28--pin DIP socket)
  DIP5 (14--pin DIP socket)
  

Memory Power Circuit

The memory chip has its own power circuit that provides it with power even when main board power is turned off. This is necessary to keep the memory from losing its contents when the board is off.

Assembly

Testing

Debugging

Check that U14 is installed correctly---the flatted side should face toward the SPRK1 component. Check polarity on C4---the + terminal should be nearer to the top edge of the board.

Memory Addressing Latch

U3, the 74HC373, is known as a transparent latch. This chip is used in the memory circuit to temporarily store, or latch, the lower eight bits of the address being selected from the memory. The 6811 first generates the low address bits, instructs U3 to store them, and then generates the high address bits and performs the memory read or write operation.

In the Handy Board component layout, U3 is mounted underneath the memory chip itself. This construction trick saves board space, and since U3 and the memory chip are highly interconnected, works out in terms of the board's printed wiring.

The only downside to this method is that U3 must be soldered directly to the board. Vertical clearance would be a problem if a socket were used. This should not cause any great consternation; as long as U3 is soldered carefully (and in the correct orientation!) there should not be any difficulty.

Assembly

Get DIP3, the 28--pin socket for the memory chip. Using an X-acto knife or diagonal cutters, remove the center plastic support from the socket so that the socket may be installed around U3. Install the socket its mounting holes, aligning the notch on the socket with the notch on the printed component outline. Solder it in place.

Testing and Debugging

Memory Chip and Decoding Gates

In addition to U3, the latch chip, the memory requires a couple gates from U7, a package of four NAND gates. (Logic gates for interfacing digital circuits are generically known as ``glue logic'' in digital design.)

In this assembly step the remainder of the circuitry for proper functioning of the memory is installed.

Assembly

Place U7* (74HC132) in the socket, aligning its notch with the one on the socket.

Place U2* (62256-12LP), the memory chip, in its socket. Make sure to align the notch on the chip with the notch on the socket/component placement marking.

Testing

Debugging