On Fri, 21 Aug 1998, Manuel Bilderbeek wrote:
> > I can imagine a system like this that would have far less problems:
> > 0V means "0"
> > 5V means "1"
> > When the level goes from 0V to 5V, at 3V the bit flips to "1".
> > When the level goes from 5V to 0V, at 2V the bit flips to "0".
> >
> > Does anyone know how things work in the MSX ICs?
>
> At my electronics course, I learned there are conventions/rules/standards for
> this. The level for 0 and for 1 is in these conventions. The most used
> conventions are CMOS and TTL. It depends on which one is used ofcourse.
>
> CMOS is 'stiffer' than TTL. But I don't know what the limits are exactly.
CMOS chips has a transfer curve infinitely differentiable, i.e., there's
no abruptous transitions. For example:
Output ^
|............
| ...
| ..
| .
| .
| .
| .
| .
| ..
| ...
| .............
---------------------------------------> Input
This means that, if the source is 5.0V, the inversion voltage of a CMOS
inverter is exactly 2.5V (theoretically). But if you want to know what
happens if you apply 1.5V to the input of a CMOS inverter, I can say that
an approximated value is 3.5V. So, for digital signals, it's strongly
recommended that the low level doesnt' exceed 1.0V and the high level
doesn't be below 4.0V, supposing that the source (VDD) is 5.0V.
In TTL circuits, the standard says that:
source voltage: 5.0V +/- 10%
maximum low level input signal: 0.8V
minumum high level input signal: 2.4V
But what happens when you apply a signal between 0.8V and 2.4V? Everything
can happen! In this range, each integrated circuit from each manufacturer
can do what it wants! It's unpredictable.
To solve this problem there are an special family of TTL integrated
circuits called "SCHMITT TRIGGER", that implements an special transfer
curve called "HYSTERESIS". The phenomenom of hysteresis was first
discovered in magnetic materials, and after the concept was applied in
conversion of analog signals to digital. It works like Manuel explained,
and below there is a transfer curve of an SCHMITT TRIGGER:
Output ^
5V |..............>......
| . .
| . .
| . .
| . .
| ^ V
| . .
| . .
| . .
| . .
0.4V| .....<................
---------------------------------------> Input
0V 0.8V 2.4V 5V
The arrows in the graphic above show that this graphic can be read only in
the indicated directions. Let's interpret this transfer curve: when the
input is very low, the output is high. Increasing the input voltage until
lower than 2.4V the output will keep the output high. If the input becomes
higher than 2.4V, then the output will go to low level, and if you
decrease the input of a small voltage, the output will keep low! This
means that the transfer curve is not linear and not continuous (so, not
differentiable). Increasing the input until 5V will keep the output low.
Now, decreasing the input voltage until higher than 0.8V the output will
keep low. If the input becomes lower than 0.8V, then the output will go to
high level, and if you increase the input of a small voltage, the output
will keep high! This is the principle of hysteresis: when a transition
happens, the input voltage should "come back" a big value!
When you asked "Does anyone know how things work in the MSX ICs", you mean
externally to the chip or internally? I think I explained externally, but
if you wanna know internally, it's a long long story!
PS: It's obvious that the network cable MUST HAVE a ground! And 9600 bauds
if only 4 times faster than a tape. So, anyone should create a network
cartridge for MSX. And why not a wireless network?
Best regards!
Marco Antonio
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