INMC 80 News


February–April 1981, Issue 3

Page 53 of 55

A word of warning about the action of the 74LS74, it fooled me (fortunately I know Dave Hunt’s phone number, and he provided the answer). The 74LS74 circuit is cleverer than it looks, it’s primed by the RESET signal, and then counts the first M1 signal it sees, it ignores the rising edge of the first M1 signal, but the output is tripped on the falling edge of the next M1. This occurs before the next address from the Z80 has stabilized on the bus and not after, as I thought.

The selection of the required jump address is made by forcing pins 3, 6, 10 and/or 13 low (that is, connected to 0V), or leaving the connections open so that the 10K resistors pull the inputs high.
A12 = pin 3   A13 = pin 6   A14 = pin 13   A15 = pin 10
Personally, I use a DIP switch here, but permanent links could be used. You could use this circuit to jump straight to Basic on ROM (at E000H) if you wished. If you want to move the EPROM area and the video up to F000H (for CP/M, or other uses, then this is simply accompished by selecting P4 to 12 on the RAM (A) board. This allows MEXT to pick up an F000H decode on ‘reset’. Obviously, this may be any one of the 16 possible 4K boundaries allowed by the RAM decoder.

Parts list

174L8367 (IC3 from buffer board)
316 pin dil sockets
6″16 way ribbon cable
1pc Veroboard
216 pin dil header plugs
114 pin dil socket
14 pole, dip switch (optional)
410K resistors

(Ed. – Gemini Microcomputers have brought out a small kit based on this circuit and it uses a wire-wrap socket to plug into the buffer board. Price 10.00 + VAT and available from your distributor.)

Page 53 of 55