I've built up the first part of my prototype. It's just an address decoding GAL and a FLASH chip hooked up to expansion port, which should give me a 256KB ROM board.

However whenever I boot up, I just get a crash. Almost exactly like I had with the EXDOS board with the slow EPROM giving me a green screen or other odd crashes.

How does EXOS work with extension ROMs? Does it look for EXOS_ROM as the first 8 bytes, and if they exist, jump to that ROM on bootup? I'm wondering whether its crashing trying to execute the ROM (I put EPPLUS on there), or if the bus is just getting corrupted through some timing issue.

Also, is it possible to page in a ROM and read its contents easily from BASIC? It would be handy if I could program the ROM with just some text, and try and read it on the EP to see if I've made any wiring mistakes.

Perfect, thankyou Zozo!

I tested the 16KB page with a psudorandom non-repeating pattern, it all verifies ok from basic. All other segments on the flash read 255 as expected. Very weird it wont boot from it... 

Ok, I'm using page 0x40 at the moment, so this should boot on any EXOS version.

So, it looks like the issue is with the M1 read cycle. I had just been using address decode & MREQ for the CE, as OE and CE need to be low for the data lines to be driven, so I shouldn't need to worry about RFSH. But to be sure, I added the RFSH line and changed CE to be address decode & MREQ & !RFSH. This made no odds...

So I had a look at the CE and OE lines with the scope. They look fine, both CE and OE assert and de-assert at the same time and I can see pulses of about 300ns (the M1 cycle) and 500ns (normal read). They look fine... 

The only slightly odd thing is if I look at CE and D0. I can see CE assert, then shortly afterwards D0 is asserted either high or low. When driven low, after CE is deasserted the D0 line tristates and floats up to about 2v. This is what seems to happen for normal bus access on the EP.

Now when the flash drives the bus high, it drives to 5v (other signals on the EP are 4v), and it seems to remain it 5v even after the bus is tristated when CE is deasserted. So that's a little strange. This is the only thing so far that seems odd, though...

Has anyone tried a 29C020 (or other 29Cxxx) with the EP? I just happened to have one lying around, hence the prototype was built with it...

I didn't read all your comments but the last several. Are you using a pull-up resistor on your extension card? Sometimes pull-up resisors can help to close the end of a bus. With this solution the floating bits can be forgotten. Of course it's just an assumption.

This is really weird. All the timings look fine. There is about a 40ns delay from the falling edge of T1 to the FLASH CE, then about another 50ns delay from there to the data being valid. So only about 90ns from the falling edge of T1 and the data is valid. And it stays valid until the end of T2 + 40ns. The CE is low for the full 375ns, just delayed. So it all looks spot on... 

The address lines also don't change until the rising edge of CE, which is after the data should have been latched.

I'll try and hook up the logic analyser tomorrow and see if I can see what's on the data lines when CE is low...

As I understand, Wifi SD cards work(apart as a storage) as an access point in which, once connected, you can read or write the files inside the card. Thus, is not the EP which operates the Wifi side, it works by itself. Even some wifi SD-cards act as a repeater of another wireless point, and then the PC, phone or tablet not lose access to the Internet.


It acts like a wireless host connection, you don't have to constantly extract the SD card and put it on a card-reader near a PC, you manage the SD files confortably on a drawer inside the PC(or phone or tablet).

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Según yo lo veo, Las tarjetas Wifi-SD funcionan(a parte de como almacenamiento) como un punto de acceso, en el cual, una vez conectado, puedes leer o escribir los fichero que hay dentro. Por lo tanto, No es el EP el que opera la parte Wifi, funciona por si misma. Incluso algunas funcionan como repetidores de otros puntos de acceso inalambricos, para que el PC, smartphone o tablet no pierdan su conexión a Internet.

Actúa como una conexión host inalámbrica, no tienes que constantemente sacar la tarjeta SD e introducirla en un lector de tarjetas cerca del PC, tú manejas los ficheros de la SD confortablemente dentro de una carpeta en el PC( o smartphone o tablet)