Electron Clone v2
I have been considering for some time some changes (improvements!) to add more features and to reduce the chip count. The chip count is roughly the same as a BBC Master!
Here are some of the changes I've been thinking about:
Add:
A joystick Port - possibly not?
Just after playing with an SD card adapter - MMFS/2
Include the MRB directly on the PCB
USB power ports directly on the PCB
Remove:
The UHF modulator
The Cassette interface
Remove composite video port
Remove the audio port (I added)
Remove the Printer Port?
Expansion connector.
I need more PCB edge real estate for connectors but I also want to reduce the overall size of the PCB.
To shrink the PCB size I could reduce the number of IC's - I could replace approximately twelve 74 series with three PAL IC's. Nine less..
And to add to the mix a new ULA replacement has been announced on Stardot. To prototype these changes I am building a Clone v1.2 board to try out some of these changes particularly the fitting of the ULA and the IC reduction ideas before committing to a new PCB.
Looking out the major parts to start building the test board and also looking at chip reduction.
Starting to solder in the resistors and then the IC sockets.
Creating the 68 pin socket from single in line socket strip using off cuts to ensure all pins align.
Adding in the oscillators and the RGB socket.
and the power connector, the keyboard socket and sound stage.
I'll add a few more capacitors then I'll stop here until the new ULA is available so I can check the height of links etc. anything that may end up under the ULA daughter board.
The new ULA arrived and sitting it on the socket I checked the component heights surrounding the ULA to ensure I installed suitable parts. Only one capacitor required to be changed from its usual part - an electrolytic capacitor. Looking at the original issue 6 drawing it should have been a tantalum capacitor all along..
Now to start testing. I removed the ULA and checked the 5V supply and that the oscillators were running. I didn't bother to check the frequency, I just used a logic probe to check they were switching. Before risking the new ULA I used a spare original to check all is ok. it wasn't of course! I had one faulty RAM IC - it's always a hit & miss when buying 'new' RAM chips. Replacing that I got the sign-on beep and the Acorn banner on the screen.
Now that all the IC's are plugged in I noticed the new ULA wouldn't sit right down into it's socket as all the IC's are now in sockets. I simply trimmed a few pins on one of the the new ULA's headers and it seated perfectly.
The time has arrived and with great anticipation I switched it on. Perfect, so I did the obligatory 'Hello World'.
Four or five years ago I did some simple benchmarks on various configurations of Electrons and BBC's so it came to mind to see how this one performs.. Slightly faster that an original Electron and in turbo mode a bit faster that the Slogger MRB.
After that little success I went on to build the rest of the board, IC sockets first then decoupling capacitors and lastly insert all the IC's. At the moment I'm one IC short and that's the 74ALS163 which is used in the Mode7 adaptor. I decided to try the 74LS163 from the Plus 2 circuit (no spares) as it only affects the Plus 2 user ports which I can test later. This allows me to power up and see what works.
And immediately I noticed the start up banner for the Plus 1 ROM is missing. So trying *H. the ROM is present and *ROMS shows all the ROM's are there so the PLUS 1 ROM works OK, it's just the banner not showing. Something to look in to - and I did. The Mode7 MK2 ROM either suppresses the banner or it over writes it - not a problem.
I then plugged in a couple cartridges, the Slogger Pegasus an a ROM cartridge with View and Viewsheet. Interestingly the Pegasus banner is displayed.
To be continued once I receive the 74ALS163...
A little aside:
While all this is going on I'm watching the power used by the system. I'm using a bench supply set to 15V to power the Electrons own power supply so the power reading can be used to calculate how much current at 5V the Electron is consuming (allow a little or a lot leeaway for efficiency). This is going to be interesting when trying out different cartridges.
The basic Electron - approx. 932mA and the Electron with the Plus 1 & 2 - approx. 1.15A - calculated 5V current.
The Electron with the Plus 1 & 2 and the Pegasus Disk interface and ROM card - approx. 1.37A. The second photo shows a meter in line with the Electron 5V PSU connector displaying the actual current drawn by the board (839mA - 4.2W) and the PSU shows the Input current (420mA) & power (6.20W). This setup is very inefficient!
This is put on hold until the system is fully functional.
In the mean time I'll take a look at making a small plugable PCB with a PAL IC to replace the first group of four IC's - IC's 13,15,17, and 18. It will also reduce the number of decoupling capacitors. I may be able to remove all these IC's anyway if I remove the composite video and UHF outputs.
