New Bus Expander by Wilco

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Revision as of 16:03, 24 January 2019 by Gflorez (talk | contribs) (How to use it)

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New Bus Expander
New Bus Expander in action

The original plan

Based on the adverts of the "ELAN" period, we can see how the original Bus Expander could have been. The futurist design was based on stackable modules like boxes that would be connected on the Expansion connector. Every module would have its own power supply.

ElanComputersLTD-4.jpgElanComputersLTD-5.jpg ELAN.jpg

These designs didn't pass the mock-up phase, where only hollow cases, but it is very interesting the idea of the flexible connector on the bottom picture.

Only the Extension port cap still reminds this early design.

The predecessors

The card presented here is a modern copy of the Hungarian made Bus Expander [1] 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.

Expansion-Unit Solder-Side.jpg

Actually we know that Enterprise Computers LTD. also reached to full design a "real" original Bus Expander [2]. It never became produced.

On the related information that has surfaced, we can see that the slots had eight pins (74) more 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 near 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 a logical solution, due to the consumption hungry components available at 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, in practice, 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 extensions where made for the Mészáros' Bus Expander, some of them are pictured on the EP128 Museum [3].

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 [4].

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.

The card

The present design was commissioned by the Spanish Enterprise-Forever member gflorez to his friend, also Spanish, 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 the German EXDOS+Floppy combo, the Microteam, Pear's compact EXDOS or the Spanish EXDOS designs.

The 3 slots on every Bus expander card share the +5V line, but for compatibility the classic cards can also be 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

Before making any change on your computer chain of expansions, please disconnect the main unit and accessories from power supply. This procedure will save you most of the problems that can affect a computer.

To connect the Bus Expander to your computer simply disconnect ANY accessory attached to the right side of your Enterprise. Then connect it to the male EDGE adaptor at the rear of the Bus Expander card, and finally connect all the set to the computer by the female EDGE connector at the left of the Bus Expander. Once done, switch On you computer and it should work the same than before. If not, turn Off the computer, clean the male EDGE contacts surfaces and try again. Also check that the EDGE contacts are correctly confronted.

Once all the equipment working, you can try to plug expansion cards on the slots(remember: always when switched off). But there are several expansion card classes, different basically by the connector used. Classical expansion cards come with a male border EDGE connector as plug, but female EDGE connectors are increasingly scarcer as time passes, so cheaper and easy to find DIL connectors have being installed on this New Bus Expander. Still there is the possibility to solder female EDGE connectors on the Bus Expander, but then they must be provided by the user.

Also, it is very easy to adapt a classic card expansion to the New Bus Expander, only soldering a double row of pins to the male EDGE connector, as they share the same step between contacts. The expansion cards must be always connected with the components side towards the computer.

But all the classic Hungarian expansion cards have its own 7805 regulator and they will not work directly on this Bus Expander, as it will not provide +9v as standard on 33A/B pins . You need to do a modification at the bottom side of the New Bus Expander. Near every slot you will find a selector made with cooper pads. Please, put a drop of tin on the selected option.

Bus Expander bottom side

Other problems may appear when the computer is turned on. 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.

Even if you have a Pear's EPower module[5] installed on your Enterprise, it will not bring great benefits to the Bus Expander, because the voltages produced are isolated from the outside of the computer as a protection design. But being more efficient, the EPower consumes less from the +9v line, and the Bus Expander can take advantage of that surplus.

Also, every 7805 regulator can feed up to 1A of current(wasting a lot of +9v current as heat), 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 their own +5V. The rear connector also doesn't count, it only draws from +9V.

To overcome this problem 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 but half the power):


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.

Pull-up array holes