New Bus Expander by Wilco
The original plan
Based on the adverts of the "ELAN" period, we can see how the original Bus Expander could have been. It was based on stackable modules like boxes that would be connected on the Expansion connector. Every module would have its own power supply.
These designs didn't pass the mock-up phase, where only hollow cases, but it is very interesting the flexible connector on the bottom picture.
Only the Extension port cap still reminds this early design.
This card is a modern copy of the Hungarian made Bus Expander designed long ago by Gyula Mészáros.
He was an active Enterprise hardware designer in an era when Hungary was behind the Iron Curtain, almost a hero when information and electronic components where very difficult to obtain.
Actually we know that Enterprise Computers LTD. also reached to full design a "real" original Bus Expander. It never became produced.
On the related information that has surfaced, we can see that the slots had eight additional pins (74) than the computer expansion connector (66), three of them used to codify the Z80 ports and the memory segments used by the plugged card, to not collide with the ports and memory segments used by the other slots in the Bus Expander. A similar technique was used on the MSX computer cartridge slots. All the important signal lines where buffered to protect the computer internal circuits and, as a curiosity, the Colour Input Bus was only connected to the first slot, the Stereo Sound Input only to the first and second. Also, only the +9V line from the computer was used, being other voltages isolated. The +5V, +12V and -12V where provided by an "inbuilt power supply" on the Bus Expander. It hadn't a rear connector like the Mészáros' design, but had terminators on the last slot.
Mészáros source of information to make his Bus Expander was only extracted from the construction of the original System Bus Bridge, in reality a single slot Bus Expander, a cheap solution to connect the EXDOS module, the only full expansion that was commercialised before the end of the company. Of course his design lacked some of the characteristics of the original design, like the extra coded pins or the +12V and -12V lines. The 8 extra pins of the connector were left with the same values than on the EXDOS interface or not connected. The signal buses where also buffered slightly different, and the +5v line was not provided so, every card plugged had to create its own +5v voltage, which was logical, with the consumption hungry components in that time.
As a result of that design, all the slots had the same priority, leaving to the extension card designer the obligation to know which resources had been used previously on other cards, to not collide with them.
We can think that the original design was superior, but practically the Mészáros' system is what has worked all these years as a standard. It is too late to take over the original design and, on the other side, the de facto "standard" has also some benefits, like no restrictions on the number of segments or Z80 ports that can be used on a slot.
A lot of card extension where made for the Mészáros' Bus Expander, some of them are pictured on the EP128 Museum .
Curiously, the Vidoton or TVcomputer, the Hungarian brother of the Enterprise, was created with a different system of coded slots integrated on the main board .
There is another totally compatible redesign of the Hungarian Bus Expander made by Pear, called the FlexiBridge, with all the signals on the connector buffered.
This present design was commissioned by the Spanish Enterprise-Forever member gflorez to his also Spanish friend Wilco.
It has some particularities. Instead of being a long card with a high number of slots like the original or the Mészáros' designs, we have opted for a modular design as a train, a main board with the buffers and only 3 slots, with the possibility of more 3 slots "wagon" cards connected at the tail connector. Every additional card has its own +9V connector and +5V regulator. As a bonus a male EDGE connector can be plugged at the rear to take advantage of existing direct connection EXDOS controller cards, like Microteam, Pear's compact EXDOS or the Spanish Wilco EXDOS design.
The 3 slots on every Bus expander card share the +5V line, but for compatibility the classic cards can be also feed with +9V.
Female EDGE connectors are increasingly difficult to get, so it has been chosen to put double holes in the PCB, to allow the assembly of EDGE or DIL connectors at will. Is very easy to convert a classic card extension to DIL, only soldering a row of pins on the male EDGE connector, that have the same thread at 2,54".
Pins 34A/B (not connected) have been selected as a form of gender protection, so it will be a good practice to block these holes on the slots and to leave that contact without pin on the extension card.
The rear connector is a copy of the expansion port, but with buffered lines. It lacks +5V at 4A/B (N/C) and has +9V on 33A/B. The +9V line is shared with the computer and all the possible cards connected in a chain, but every card have its own +5v regulator, power connector and a protection on a cooper track(a narrowing).
Throughout the design the rules of the greatest simplicity and the lowest cost have been followed, using modern components like SMD to make the card more compact. The 7805 regulator is still a cost effective way of producing +5V and so it has been maintained.
How to use it
I want to give you some recommendations of use for the Bus Expander on its second version, but if you find difficult to understand this, please ask Zozo about how to prepare the Bus Expander to work with your classic card expansions.
This card has a particularity over Mészáros' design: by default the cards plugged in the slots will feed its +5V from the 7805 regulator fitted in the Bus Expander from pins 4A/B of the connector.
But all the classic Hungarian expansion cards have its own 7805 regulator and they will not work directly, so you have to do a modification on the bottom side of the Bus Expander. The slots come unconnected from +9V on the 33A/B pins of the connector, but there are a selector at the bottom side on every slot to permit this only joining two cooper pads with a drop of tin. Then, the modified slot will be only used for a classic expansion.
Previously, you have to solder a DIL row of pins on the EDGE male connector of your classic card expansion. I will distribute the Bus Expansions only fitted with DIL connectors. But there are on the card holes to solder EDGE connectors, not provided by me.
The Enterprise power supply comes on +9V with 2A or 1,3A variants, so, when all the equipment is connected, if you experience undulations on the screen, probably you need to connect a secondary power supply on the Bus Extension auxiliary power connector.
Also, every 7805 regulator can feed up to 1A of current, and this can be insufficient to feed the three slots when all of them being populated. This doesn't applies to slots modified for classic expansions as they produce its own +5V. The rear connector also doesn't count, it only feeds from +9V.
To overcome this I recommend to substitute the 7805 regulator that comes installed (the chip with screw and heat sink) with a 2A TracoPower regulator (I don`t recommend the 1A variant, of similar price): https://hu.rs-online.com/web/p/kapcsolas-szabalyozo-es-kiegeszito/1616642/
The chip has the same pinout than the 7805, and I have used it for decades on my Enterprises without problems. This chip works at ambient temperature, is more energy efficient and doesn't burn your fingers...
On this second version of the Bus Expander, holes have been added to fit an array of 4,7kΩ pull ups to the Data bus, to overcome some problems of shadow segments when a Ram expansion or Turbo mode are fitted to the computer. You can see the pads zone near the expansion connector.