I've had some feedback from a professional electronic engineer, who told me that the PORT1 chip wasn't needed because the FTDI's data bus is tri-state.

So I have altered the board layout, and also added a resistor network to pull up the status lines, which tri-state whenever the FTDI chip is held in /RESET, which happens whenever the USB cable is not connected to a PC.

Here's the current layout ...

Just one idea: Is it possible to add 512K Flash ROM? To store the USB driver and other ROMs.

Just one idea: Is it possible to add 512K Flash ROM? To store the USB driver and other ROMs.

JLCPCB do not offer any modern surface-mount 5V 512KB flash ROM chips, so I can't just add one to the board.

They do list one type of 128KB flash ROM, but it is never in stock and available for manufacture, and so there is no point in even trying to add that to my design.


It might be possible to add a DIP socket to the board, but it would make the board significantly larger.

Even then, I am not sure that I could add in a 32-pin socket for a 39SF040/27C040, but it might be slightly more reasonable to add in a 28-pin socket for a 64KB 27C512, which would be plenty of space for a driver and a couple of ROMs.


Now having said that ... what driver are we talking about?

I can't imagine the USB serial port as a boot-time bulk-storage replacement for either the SD Card Adapter, or the MicroTeam floppy disk interface, or the Symbiface 3, and all three of those existing hardware boards already have plenty of unused ROM space that could be used for such a driver, if someone wanted to write one and create something like a "DriveWire".


If you want to change the EXOS "SERIAL:" driver to use the USB, then that might be more sensible to do inside the EXOS ROM itself, especially because there is still 6KB of unused space within the EXOS 2.4 ROM.


If you are really talking about having some space for existing ROMs, then do people really want that?  If so, would 64KB be enough?  Yes, I know that there are more than 64KB of Enterprise ROMs, but are they useful ROMs that people need to be permanently inside the Enterprise, or can they just be loaded with EPDOS's "LROM"?


Oh, one more thing ... since I am very new at this, I am also worried about adding too many new device loads onto the Z80's memory and data buses.

It might be possible to add a DIP socket to the board, but it would make the board significantly larger.

Even then, I am not sure that I could add in a 32-pin socket for a 39SF040/27C040, but it might be slightly more reasonable to add in a 28-pin socket for a 64KB 27C512, which would be plenty of space for a driver and a couple of ROMs.

I've spent a lot of time trying to reorganize the circuit board to add either a 28-pin or 32-pin DIP for a ROM, and it just isn't working out well.

I think that it is probably best for me to go ahead and order just a couple of boards with the current design, and make sure that it works properly.

Have you tried to use the bottom side to put the Eprom Socket alone? There is enough space for it.

Yes, I did consider putting the flash ROM DIP on the bottom of the board, but it doesn't help because you still need through-holes for the pins, and the surface-mount RAM chips are the same width as the flash ROM, meaning that you can't interleave the parts on the board, and so you still need to move the SRAM chips out of the way by exactly the same amount that you would if you put the flash ROM on the top of the board.

Or it can be totally soldered, on PLCC or TSOP encapsulate, but then you need to take the /WE signal from the EP mainboard. Zozo can modify his SD cartridge updater software.

So you mean like this board ...

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The problem with that, is the question of who is going to add those components to the bottom of the board?

The PCB manufacturer will not do it, and I am certainly not capable of hand-soldering either an SMT PLCC socket or a TSOP package.

Neither are most of the people here, I suspect, or they would have already built their own boards.


Replacing the USB components with a through-hole PLCC socket is something that I can imagine would work, and be something that probably more people could solder on at home because it is 0.1" spacing ... but I have no interest in that myself, because having the USB is the most interesting part of the board to me.

For anyone wondering what has happened with this ... I got sidetracked with another project, and when I was ready to come back to this and place an order for production, JLCPCB had run out of stock of the FT245R chips.

I keep on checking regularly to see if the part has become available again, but with no luck.

I don't know if this is related the global shortage of components that everyone is complaining about, but I don't think that there is much that I can do, except to keep on waiting.

That's annoying. Hope that stock comes back soon.