This board is now supplied fully built and tested - no soldering needed
This is an interface for connecting a Raspberry Pi Zero Processor to the internal co-processor slot on the Acorn BBC Master 128. Only the interface itself (with the surface mount components in place of course) is included.
The Pi's interface pins operate at 3.3 volts logic levels whereas the Master uses logic powered from 5 volts. The two different signals cannot be connected directly together, hence the requirement for an adapter. Adapter boards such as these are sometimes called 'level shifters' because they shift logic levels from 3.3V to 5V and vice versa. Simplistically, that is all the adapter does.
It differs from similar products in that a four-layer board has been used. Such boards are more difficult to manufacture and therefore slightly more expensive, but have the advantage of much larger and therefore lower impedance power and ground planes.
There are five main decoupling capacitors whereas some designs don't have any.
The two main level shifter logic chips are also different. The ones used here require two separate power supplies, 3.3V and 5V. However, rather than draw current from the Pi's 3.3V pins, this adapter has a separate voltage regulator on the board. Two major chip manufacturers both recommended the use of the dual supply chip in preference to the single supply devices used on similar designs.
The Raspberry Pi itself can emulate several different processors, including two speeds of 6502 3MHz and over 250MHz) and Z80 to name just three. These different processors can be selected with a simple software command. Most users will probably choose one of the 6502 options for the simple reason that their software is likely to be BBC BASIC or 6502 machine code.
Using with a Raspberry Pi 3 A+
These are supplied with the 40-way GPIO pins already soldered in place. As the pin connections are compatible with the Pi Zero, it follows that the 3 A+ can be used instead if preferred. There is no need for a special "Raspberry Pi 3 A+" adapter. The socket used on the level shifter results in a very firm connection with both the Pi Zero and 3 A+. The fact that the 3 A+ board overhangs the level shifter is of no consequence. The last photo shows the Pi 3 A+ installed.
What's included?
Included with the adapter board are plastic clip-in pillars for supporting the Raspberry Pi Zero processor. In fact, the Pi is a fairly firm fit in the 40-way female connector on the adapter board and the pillars are unlikely to be needed. The rationale behind including them is that obtaining them separately is often quite expensive - you may have to buy a minimum number and then pay a handling fee and postage on top.
The Raspberry Pi Zero processor itself is not included but is easily obtainable. You need the version with a set of header pins connected to the GPIO port. You can either buy the basic Pi without the pins and fit them yourself or, slightly more expensive, with the required pins already in place.
The second photo shows the two versions of the Pi, both with and without the GPIO pins fitted.
Note also that the Pi will require a micro SD card with some freely available data files loaded on it. The Micro SD card itself is not included but details of the required software are supplied with the kit.
The third picture shows a typical completed installation in a Master 128. The point is made again that neither the Raspberry Pi is included nor, unsurprisingly, is the Master itself.
Guarantee
The product is covered by a 1 year guarantee. Additionally, most eBay Buy It Now sales are covered by UK consumer law. In essence this gives you a 30-day period in which to return the goods for any reason whatsoever. If you wish to return this item in the 30-day period then you will receive a full refund. The cost of returning the item to us is to be borne by the purchaser unless, of course, the item is actually faulty in which case we will refund any reasonable postage costs as well.
For UK buyers this offer is extended further. You can return the item in good condition for any reason whatsoever within 30 days for a full no-quibble refund. Not only do we refund the full amount paid but also the return postage (first or second class standard). What do you have to lose by checking it out?