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  My Mephisto Risc 1MB II
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   Author  Topic: My Mephisto Risc 1MB II  (Read 995 times)
Kostea
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Re:My Mephisto Risc 1MB II
« on: February 14, 2011, 06:13PM »
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Hi Danox,

Haven't opened any Mephisto boards yet, and don't own a RISC module...yet.

Let's list the CPUs used in the modules for the Mephisto boards:


6502
- Polgar, MM modules

68000, 68020, 68030
- all Richard Lang modules

SH7034
- Magellan (Frans Morsch)

ARM2
- RISC 1 and 2 (Ed Schroeder)


All the above CPUs and modules have one thing in common, they all need stabilized 5 Volts to run.
The only difference is the amount of Amps they require.

The simplest 5v stabilizer was done with a 3-terminal 7805 integrated circuit, which looks like a power transistor. It can carry a maximum of 1.5 A when installed on a big heatsink.
There are other integrated circuits from the LM series which can take a maximum of 5-10 A. These LM chips need a very small heatsink or no heatsink at all under 2 Amps. I would still mount them on a heatsink just to increase the reliability of the whole thing.

Now because all modules need 5v, it made perfect sense to put a 5v stabilizer in the chess board instead of having one in each module because:
- there is more room in the board than in a module to build a massive heatsink.
- heatsink inside a module would have radiated heat to the other chips around the heatsink
- the modules would have been more expensive.

All the modules (except RISC) required just a 'regular' 5v stabilizer providing maybe less than 1 Amp.

More power hungry is the ARM2-based RISC 1 and 2. I cannot find the datasheet of that ARM2 CPU, and can't tell how much milliAmps it uses. Apparently the 'regular' 5v stabilizer in the chess board  was not strong enough, therefore it was powered through a DC jack in the module itself.

It seems that your board was modified and retrofitted with a stronger 5v stabilizer capable of providing more juice/Amps suitable for a RISC module.

A good question about your board would be if it's compatible with the pinout of all the other modules except RISC. I mean, if it provides 5v on the same module pins as the 'regular' unmodified boards do.

One other plastic chess computer, the Saitek Risc 2500, uses exactly the same ARM2 CPU as the RISC module.
In this case the stabilizer is inside the external power adapter. It uses 5.4 V to run. The 0.4 V voltage increase is due to the way the Risc 2500 is powered on. It does not have an ON/OFF power switch. That function is accomplished by the GO/STOP button. When the 5.4 V power supply is connected, when pressing the GO/STOP button, a PNP 10 Amps power transistor (BD912) inside the Risc 2500 is switched on by providing ground to the 'Base' terminal. This way the power transistor opens a current flow path between the Emitter and Collector terminals. The +/positive voltage then is allowed to flow through the Emitter-Collector junction of the power transistor. Because the transistor is a semiconductor, it opposes the flow of electricity, basically even if the Emitter-Collector junction is fully opened it still has a resistivity. That small resistivity multiplied with the electrical current flowing thru transistor creates a 0.4 voltage loss, which has to be accounted for.
5.4 - 0.4 = 5 v provided to the CPU and all integrated circuits.

A semiconductor is not categorized as a full conductor, that's why it's called 'semi'. It has a higher resistivity than the 'conductor' metal types: gold and silver, even higher than the copper and aluminum.

Mephisto Montreux uses the same 'power on/off' scheme as the Risc 2500, but it has a different ARM CPU. It uses 5.6 V in order to account for the bigger 256kb ROM chip which uses more mA than the 128kb from Risc 2500.

There are several other older washboard design chess computers from Saitek who used the GO/STOP stuff. They still use that in the current Mephisto Master. The ones with an H8 CPU simply either 'wake up' or put the CPU to 'sleep' when pressing GO/STOP. The H8 CPU has a sleep mode, that's how it simulates the ON/OFF (ON = wake up, OFF = sleep). The H8 CPU still gets 5v while in sleep mode, it just won't run, creating the illusion that the power is off. The only time the power is really OFF is when the power adapter is unplugged.
The ARM2 CPUs didn't have sleep mode and had to use a power transistor for ON/OFF and then increase the voltage to 5.4 or 5.6 to account for the voltage loss over the power transistor's Emitter-Collector junction.

Regards,
Kostea
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danox574
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Re:My Mephisto Risc 1MB II
« Reply #1 on: February 13, 2011, 08:46PM »
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To answer my own question, I have disassembled the Exclusive and found a 5V regulator installed with a very large heatsink in the chassis to deliver 5V instead of 9V to the module bus when the 9V adapter is hooked up.  I can understand how the other modules would not work because the installed modification will always reduce the delivered power to 5V.

Maybe someone here did the work?
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danox574
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My Mephisto Risc 1MB II
« Reply #2 on: February 13, 2011, 08:27PM »
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I have a Mephisto Risc II in an exclusive board I purchased recently.  It's very odd though -- the board (non-battery type) is powering the RISC module from the power input on the side of the board, not from the power on the module.  The adapter is a 9V adapter, although the specifications on the Schachcomputer Wiki list this as being a 5V adapter (HGN5050).  It's also prominently discussed that the RISC module cannot be powered from the board side power input, yet that's how the last owner always used it and it seems to be working great.  The RISC module has been completely reliable.

Does anyone know why this is?  I understand that modifications were carried out on some boards to allow this, and apparently it doesn't allow other modules to be used (I have no other modules right now to test).    Is there any information online about modifications performed that might have been done to this board?

Thank you as always!
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