Sequencer to PCBs

Now the necessary changes to original sequencer have been done and tested on breadboard, so it is time to start designing the printed circuit board(s).

The sequencer uses mostly standard 7400- and 4000-series ICs. They are not very hard to get, I have many of them in my own stock, and local Hacklab has good supply of vintage components. The 74C922 keyboard encoder is a bit more rare, I ordered it from eBay. And soon after that Hacklab got another component donation, including bunch of 74C922s. Such is life.

Other than that, there are couple of 555, ULN2003 and the RAM, which I already mentioned in a previous post. Nothing exotic.

As previously said I’m using Eagle free version for designing PCBs. It has some limitations but they are not that bad as they look. The sequencer contains 24 ICs, so there is no hope in fitting them to one 8*10 cm board. But two boards might be feasible, it is only 12 ICs per board. When stacking those with spacers, the stack will be much more space-friendly inside the enclosure than one larger board.

When dividing the ICs to the two boards, main goal was to minimize the connections between the boards. In my opinion, I managed it quite well: only 9 signals + supply and ground from board to board. By putting a 10 pin pin header/female header pair to the boards, the board stack is also easy to separate.

There is only one 5 V regulator which supplies both boards. The input voltage to the regulator will be something like 15 V, so there will be quite a big voltage drop and hence power dissipation in the regulator. But the HC- and 4000-series ICs do not need that much current, quite small heatsink will be enough, according to my tests on breadboard.

In this kind of digital boards I usually try to connect most of the cabling with connectors. This makes the cabling neat and modular and easy to disconnect if needed. Pinheaders and corresponding female ones in the cables are normally my choice.

After some drawing and thinking, I came up with this kind of allocation of the functional blocks between the boards:

  • Sequencer board 1:
    • Led + step oscillators
    • Led + step counters
    • Address muxes
    • Step button encoder
    • Led control
    • Run/stop toggler
  • Sequencer board 2:
    • Memory + latch
    • Bit inversion XORs
    • Channel encoder + mux
    • Pulse generator monostables
    • Trigger pulse latches
    • 5V regulator

Boards will not be very tight, but tight enough so that it makes no sense to try to etch them home. They will be two sided, have quite many vias and so on. This time I will get them made by a PCB factory. I usually use Elecrow, they are doing neat job. This “factory or home etching” decision must be made before designing the layouts.

 Again sitting some hours with Eagle, and designing:


 

The resulting schematics and Eagle files are in my Github.

After couple of weeks waiting for Elecrow’s shipment, the boards arrived. Elecrow ships normally quite fast, but the cheap transport from China to Europe is not the fastest. But I'm not in hurry with these projects, there are no deadlines.


And after a couple of hours of soldering, the boards look like this:


If someone is wondering how I decided, which ICs are SMD and which are DIP: I did not. They are what I happened to find.

Here is my quick-and-dirty test setup for testing the sequencer, it simulates the buttons and leds.

Here is a drawing documenting the various connections related to the sequencer: buttons, leds, tempo pot and others.


Now also the sequencer part is ready, next phase will be deciding about the enclosure.


Comments

Post a Comment

Popular posts from this blog

Secrets of the YM2154

Image
  After a break, I returned to the PSR-70 reverse engineering project: t he YM2154, a.k.a. RYP4 is still an unknown territory. The time has come to reveal its secrets. R YP4 is used a bit more widely than OPQ. Besides PSR-60/70 it is used at least in PSR-80 and RX- 11/15/21 drum machines. This time I did remember to check for service manuals, and found one for RX-11: https://elektrotanya.com/yamaha_rx11_sm.pdf/download.html . This contains a full schematic and other useful information.   T racing register writes Because I had good results with OPQ by tracing what the PSR-70 firmware is writing to the chip registers, this would be a good starting point also for RYP4. The chip has 128 registers which reside in Z80 I/O address space at addresses 80H...FFH. Looking at the disassembly listing reveals quickly that there is no single function which would take care of all the register writes, like OPQ had. There are many IN and OUT instructions accessing the RYP4 register
Read more

Reversing the PSR-70 hardware

Image
  First step in the actual reverse engineering is to start studying the PSR-70 hardware. This is quite service-friendly (meaning hacker-friendly) device: all circuit boards are single-sided and copper side silkscreen has IC numbers and jumper wire positions marked. There are also many useful looking signal names marked, like “D7”, “A12”, “IOWR” etc. It would be possible to draw the whole schematic from one high-quality photo of the circuit board. My intention is not to draw the whole schematic, but at least the address maps should be figured out. It will make the interpretation of disassembly much easier. All address decoding is done with 7400-series discrete logic, no PAL-circuits or other nuisances used, this is very good.   Block diagram Most of the electronics is on the large main board . The other cards are: Power + audio board contains the audio amplifiers, some kind of analog chorus-effect based on a BBD-circuit, power amplifiers feeding the speakers, and voltage
Read more

Digging into YM3806

Image
In the previous post I got the PSR-70 original firmware modified so that it dumps to serial port all register writes done to OPQ chip (YM3806). Now I’m getting the data I wanted and plenty of it! A simple keyboard key-down or key-up produce only under 20 register writes but changing sound from the front panel results in about 300 writes. Why 300, there are only 256 registers? Yes, but there are many registers which are written several times. T he raw dumps from the modified firmware are quite hard to read, they just list the register address and written value in hex in the order they happened: 1F=20 5E=1F 56=1F 4E=20 46=20 5E=81 56=81 4E=81 46=81 9E=1F … To make some sense out of this I wrote a small python script to analyze the dump. It lists all registers in numerical order and all writes done to that register. In the script output the first column is OPQ register number, second column is the number of writes to that register and then a list of the written values.
Read more