The Nibbler 4-Bit CPU Project – The Missing Chip

Almost-fully-assembled-nibblerThe circuit board is soldered, the microcode and program ROMs are flashed so the final step before switching on my 4-bit CPU board is to put the chips into the sockets. I'm glad I took extra care when doing that because quite to my dismay one bag contained the wrong IC for the data bus driver. Instead of a 74HC244 2x 4-bit buffer with a 3-state output that is required to select either the ALU or the FETCH register for output on the 4-bit data bus, a 74HC574 was delivered which contains d-type flip-flops. Apart from having a completely different functionality, input and output pins on that chip are different than on the 244.

If I hadn't caught the mistake, a number of chips would probably have been fried at first power-on. I could hardly believe it as the invoice correctly showed a 74HC244 and also the the bag for the chip had a 74HC244 sticker on it. I'm glad I didn't trust the bag labeling and checked the number on the chip once more after having inserted it on the board.

Missing-chipsQuite frustrating to sit in front of a fully completed board and not being able to power it on due to a single component missing that is worth only a couple of cents. Fortunately, a local supplier had a 74HC244 in stock so instead of waiting for it to be delivered I went to pick it up in the shop during lunch break the next day. The second picture shows the chips I picked up in the local store. Joy for less than 2 euros! Almost showtime now!