Re: 6502 to 8501: new details

From: Marko Mäkelä (
Date: 2002-02-22 10:05:07

Daniele Gratteri wrote:
> I will try to measure voltages on the CASSETTE READ and WRITE lines.

They should be steady near GND or VCC when nothing is being written and 
the cassette drive is not playing a tape.  When something is going on, 
the readout should change to something between GND and VCC.

> Can you explain me what is a 'pull-up' resistor and how to connect one of
> these? What value has it to be?

A pull-up resistor gives a "default" value of VCC to a signal.  Connect 
a 4700 ohm resistor between VCC and the signal.  When nothing is driving 
the signal down to '0', it'll be held '1' by the pull-up.

The NMOS (n-channel metal oxide semiconductor) chips in the Commodore 
can actively pull signals down, but not up.  The opposite is PMOS, which 
is quite rare, I think.  The combination of NMOS and PMOS is called CMOS 
(complementary metal oxide semiconductor), which can drive signals both 
up and down.  In CMOS logic, the power consumption depends on the number 
of state changes, but NMOS takes about the same power, no matter what 
the clock speed is.

BTW, I just noticed that the pull-up in the C2N232 is not necessary, at 
least not with my flat C128.  I also noticed that the 33pF capacitors in 
the crystal circuit are unnecessary.  (How did I find this out?  I have 
run out of SMD caps and resistors; I only had the five capacitors for 
the MAX232.)


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