Making a register for my SAP 1 computer
Having built the clock module for my SAP 1 computer I decided to build the register module. In a change to the way Ben built his, I will not be connecting my 'B' register to write back to the bus.
Ben used 74LS172 chips with a 74LS245 to drive the bus. I didn't have any 74LS172's so I looked either for an alternative, or supply of these. I found out about AMD's AM2900 series, and the AM2918 in particular. This has 4 D-flip-flops with inverting three state gates connected to the /Q output - I could connect the 3 state outputs to the bus and the normal outputs to LED's and my ALU.
But AM2918's are quite hard to find, but I did come across some Russian KS1804IR1 copies for sale, Russian military surplus perhaps?, and decided to give them a try. Here they are (sold as a lot of 10)
These have the standard Russian pin spacing of 2.5mm rather than 0.1", but they fit into my breadboards fine.
Firstly a quick test to check they do what I hoped:
Since the SAP design is 8 bits I will need 2 of these for my 'A' register. I placed them 10 rows from the end of the board to connect the bus, since the pinout of these chips isn't convenient for wiring a bus. Firstly the power connections:
Since we will be either reading from or writing to the bus, but not at the same time, I realised I could connect the Y output and D input and simplify the design a little. Here are those connections made:
As I will be placing the ALU to the bottom of this board I left a gap of 2 rows between the IC's to route signals to a row of LED's to show the register contents, Here is that done, the yellow wires carry right side to the left side.
A more permanent set of LED's and bus wiring will be added later.
Ben used 74LS172 chips with a 74LS245 to drive the bus. I didn't have any 74LS172's so I looked either for an alternative, or supply of these. I found out about AMD's AM2900 series, and the AM2918 in particular. This has 4 D-flip-flops with inverting three state gates connected to the /Q output - I could connect the 3 state outputs to the bus and the normal outputs to LED's and my ALU.
But AM2918's are quite hard to find, but I did come across some Russian KS1804IR1 copies for sale, Russian military surplus perhaps?, and decided to give them a try. Here they are (sold as a lot of 10)
These have the standard Russian pin spacing of 2.5mm rather than 0.1", but they fit into my breadboards fine.
Firstly a quick test to check they do what I hoped:
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| Bus outputs turned off |
 |
Bus outputs active.
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Since we will be either reading from or writing to the bus, but not at the same time, I realised I could connect the Y output and D input and simplify the design a little. Here are those connections made:
As I will be placing the ALU to the bottom of this board I left a gap of 2 rows between the IC's to route signals to a row of LED's to show the register contents, Here is that done, the yellow wires carry right side to the left side.
A more permanent set of LED's and bus wiring will be added later.
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