A3000 minipodule interface

The A3000 minipodule interface is essentially a cut-down podule bus.  It only has an 8 bit data bus, limited address bus, and only a 5V power supply.  The pinout can be found in table 1.  Plugs A to D are a series of pin headers, lettered from left to right when looking at the back of the machine - the pin numbering also follows this convention.

There is not much more to be said for it.  Acorn are at pains to point out that the power supply can only provide 500mA and only 100mA of this may be consumed inside the case for thermal reasons, but much commercial hardware ignores this.

A4 network slot interface

This is an even more cut down podule interface.  It's almost the bare minimum to attach a 6854 ADLC for Econet, but has a few more signals that make it a little more general.  It is provided by a 2mm pitch 2x13 way header on the motherboard, pinout in table 2.

We have an 8 bit data bus, but only a 4 bit address bus.  Rather than a podule select, we have two signals /S2 and /S4.  These come from an address decoder on the motherboard.  /S2 space in fast mode is available from &03320000, whilst /S4 space is from &03340000.  Essentially these are podule select signals before they have been decoded to individual podules.  So effectively our 16 byte wide card fills all podule space in the machine.

The other feature of note is the /ECON signal, which is low when the 'Econet'
hardware is to be switched on.  This can be controlled via the Portable_Control SWI, and allows the system to switch the hardware into power save mode when not being used.

Risc PC network slot interface

The netslot interface is again a cut-down podule interface using a 48 pin DIN41612 type R connector - its pinout is shown in table 3.  The main differences are active high, level triggered RESET and IRQ pins (the podule bus has them open-collector), and the podule select mechanisms.  It should be noted that this is the pin numbering of the motherboard connector - the podule connector has reverse numbering (so pin 1 of the motherboard connects to pin 16 of the podule connector).

/CSNET is analogous to /EASI in allowing access to the netslot address space, which is &0302B800 to &0302BBFF.  It can do type A or type C cycles as EASI space can.

/ROMNET is rather different.  Instead of using the traditional means of accessing a podule ROM with a latch, a counter is used instead.  Figure 1 shows the arrangment suggested by Acorn.  A write to /ROMNET space (&0302B000 to &0302B3FF) will reset the counter, and subsequent reads from it will cause the counter to increment, providing consecutive bytes to be read from the ROM.  Different addresses within /ROMNET space will return different 'pages' from the ROM.  This means no podule loader is required and the hardware is relatively simple.

Psp1 to Psp8 are Product Specific Pins.  These are supposed to vary between each implementation of the specification.  On the Risc PC Psp8 is assigned to be /DACK0, Psp7 DRQ0 and Psp6 TC.  The ARM7500 doesn't provide DMA, so I assume that these are not connected on A7000(+) machines.

Iyonix podule interface

The Iyonix podule bus is essentially that of a Risc PC, but with a few more features.  I have yet to see the documentation for it, but the following is based on a conversation with Castle at the South East Show 2002.

As I mentioned last month it provides double-speed transfers in EASI space, allowing twice the podule bus bandwidth.  This is done with Podule_SetSpeed providing a further set of speeds E-H.  The bus also allows halfword (16 bit) transfers; StrongArm provided new instructions to allow these but the Risc PC's memory system didn't support them, returning an incorrect result.  This should make accessing 16 bit wide devices somewhat easier (and slightly quicker).

As of November 2002, DMA was not supported.  I'm not clear whether it has been subsequently provided.  However the bus additionally provides 3.3V tolerant I/O.  Whilst no 3.3V power rails are provided on the bus, it allows the use of 3.3V logic without having to level shift up to 5V only to have this 5V level shifted down to 3.3V again on the motherboard.



That's all folks

Apologies for my rather extended break in finishing this series!  Hopefully you should have a clearer idea of how the podule bus works, and be equipped to start building your own podules!