Open source STF clone project.
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Post by exxos » Thu Aug 17, 2017 8:56 pm

STF2.PNG (139.57 KiB) Viewed 304 times
It quickly came apparent that my first design was simply to large :( So a total re-think was in order.

This is only a "quick" layout to see how things would route. What we have now is the BLITTER, CPU, GLUE,ROM under the PSU. This space is actually wasted and no real room for any expansion card. So it makes sense to route there.

The MMU sits just in front of the PSU, the ST-RAM card is also there and the bus isolators under the ST-RAM card to save space.

The shifter is now located right next to the monitor output along with the RGB drivers.

One change which was suggested by Rodolphe was replacing the RS232 area with a MAX232 IC.
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The MAX232 did not have enough IO's so the MAX238 was used. The good thing about this IC is that it is more compact, the 1488 and 1489 drivers are no longer needed. Also the TL497 voltage generator is also no longer needed as it is built internally in the MAX238. So overall it saves on a lot of parts and PCB space.
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There is a MAX235 which is exactly the same as the MAX238, but it does not need the external capacitors. I was going to use that but its easily twice the price of the MAX238 :( So no use in spending £5+ on a IC where 4 cheap capacitors can be used instead. So the current design now uses the MAX238.

I am also now working to align the connectors to their proper locations as well as improving the IC layout. I hope this will allow me to reduce the PCB space to half the size so making a prototype will cost a lot less.

There are also a couple of resistors packs which are linked all over the place. I may remove those into individual resistors or 2 smaller resistor packs. No use in having tracks running right across the board just for a pull up resistor.

I am still considering what to do about the TTL chips. They could easily be replaced by a GAL. Though problem then becomes more routing across the board to go to 1 GAL. A lot of gates in each IC are not even used. Or can be done with less gates. There is also the slight issue that it makes the design a little more complicated as GAL's would need to be programmed instead of basic TTL chips. Though considering in the future there will be a big PLD for the booster logic. I suspect overall I will moved to GAL's instead of dedicated TTL chips.
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