Ok, that makes my life easier...

If you needed additional functionality it would of course be possible to reprogram the CPLD and PIC to add it in as well.... 

That's extremely cool! My original idea for the SD card interface was to have it internal to the Enterprise like an internal hard drive and have a way to access the SD card via a serial connection. This is just so much better! Awesome find.

I've realized that an Ethernet interface for EP can be more useful than an SD card. Why? Because it is a bit cumbersome to always exchange SD card between your PC and EP if you want to copy programs, or better: if you want to cross-develope EP stuff on PC and you need to do lots of data transfer. By contrast, if you have an Ethernet solution you can exchange data easily between your PC and with the whole Internet! You can even have some simple protocol (maybe UDP based) to access a disk image so it would be a quite normal disk from the point of view of EXDOS, just like an SD card solution, or Zozo's IDE interface, etc, just through the net. My home router is always switched on and it has an USB port. I usually keep a bigger pendrive in it (but for sure even a PATA/SATA to USB converter is OK with a harddisk, or any NAS like home stuff), and the router can share the storage via FTP or SMB/CIFS. But I can even run a custom little UDP server for EP for example (it runs Linux so it's not a problem at all to do). So the storage is always there via network, to be used even for EP.

Of course the "disk image level" access can be extended later (as Bruce wrote) to access data not so much with EXDOS but via custom protocol like SMB/CIFS with "a little EXDOS hacking" to support eg DIR on it. I wanted the same with my JavaScript EP emulator via AJAX functionality, and I named it as "NetLinkFS", the theory is similar just I wanted to avoid to hack EXDOS and I used different command set like NDIR instead of DIR, but otherwise the idea is similar.

Meanwhile, if you have Ethernet for EP, you can even load programs for the Net, you can even chat on your EP (like my iRC tries) or even sending/receiving email or some limited text based web browser expereince on EP what I've also tried in my emulator, not only accessing a disk image basically, what would be used with an SD card solution.

At first I thought about implementing a general purpose SPI bus. It would be great to access an SD card and also an ENC28J60 ethernet controller (I have that IC). However Bruce idea about that wiznet module (also Zozo mentioned that idea) is better in the sense that it already supports even TCP/IP by hardware and simple integration on the bus system of a 8 bit machine, no need for SPI at all. Also, writing a TCP/IP stack in Z80 assembly for EP would be a larger work (IP fragmentation, and other issues, especially with TCP; UDP/ICMP/ARP level is much more easy, I've already done it for Commodore 64 some years ago), and it would be somewhat memory hungry solution. That module does that for you, don't need to implement basic network protocols by yourself (of course something like SMB/CIFS should be, but it's another question, the basic of these is UDP, TCP ...).

Another thought is to use something like the PIC18F97J60 for the sd card controller chip. This chip has an in built Ethernet mac and phy and Microchip provide a TCP/IP stack. And it's cheap! Could extend the command set which talks to the SD card to include Ethernet as well...

I have no idea about PICs only AVRs, but indeed. Some MCUs even have USB (but then USB host or at least OTG would be needed) so in theory, a pin-monster and feature-rich enough MCU can give Ethernet, SPI, USB and even serial connection well other things too if needed like I2C (can be used with eg a little RTC chip to provide real time clock as well). And btw USB - maybe implementing USB storage protocol is even more comfortable than having SD card interface: pen drives, most mobil phones / mp players (which uses the generic storage protocol and not some own one), or even harddisk with USB adapters can be accessed then.

Edit: On looking further BUSRQ is tied high on the Enterprise. No DMA possible without internal modification...

Yes it is only posible if the CPU and DMA are in same CPU board and looks a "CPU" for the system. (Or some signals directly connected from the Z80 to DMA)
If my Z180 project will be successful then it is have built in DMA channels

If the network access could be over wifi, the whole thing could be internal 

If “someone else“ was handling the tcp/ip stack I wonder if you could get away with not needing to use interrupts for network access - it would be a lot simpler. At the EXOS level everything is synchronous ie the z80 would always send a request and wait around for a reply, possibly repeated several times. Even if the spare interrupt works at the hardware level, interupts always cause complications for software and there could be problems with anything from extra stack usage causing odd crashes (the network interrupt interrupting the system interrupt or vice versa depending on how the priorities work out) to problems due to the time taken copying a packet from the net h/w to the z80 in interrupt time. You'd only need interrupts if the high level protocol (SMB ?) needs to respond to an asychronously-received packet, and I don't think it will (assuming it's not being a server!).

If the wifi SD cards turn out not to do what we want, another possibility is to implement exactly what we do want and not involve the z80 in the network at all ie. write PIC s/w so you can read and write files to the SD card directly over the network.

If you go for an i/o-based solution check out how the ethernet module I previously posted a link to does it - it's use of an auto-incrementing register (effectively a pointer to the on-module memory) would (I think) allow INIR etc to be used.

One quick question on the Dave and io requests - how does the io port get passed to the address bus as a14 and a15 are controlled by Dave? The z80 passes the io port to both a0-a7 as well as a8-a15 I believe. Does Dave pass through the top two bits of the io port?

The A14-15 also go through on the paging circuit in IO mode. Then the A14-15 on the System Bus depend of the currently paged segments.

Thanks Zozo, that makes sense. Something to be aware of.

I've been looking at SpeccyBoot here --

http://speccyboot.sourceforge.net/
https://svn.code.sf.net/p/speccyboot/code/tags/speccyboot-1.5/

There's some nice code for dealing with the ENC28J60 directly from the Z80. So perhaps just an SPI interface and let the z80 deal with it, after all. I'm a big fan of code re-use, as there will be a fair bit of work on that side...