Still confused on a few Cassette Drive components (was, a long time ago: Re: CASSETTE MOTOR DRIVE Questions)

From: Jim Brain <>
Date: Tue, 25 Jun 2013 02:34:55 -0500
Message-ID: <>
Looking at some information from the list from last year, when I was 
wondering about the cassette port motor circuitry, I found I am still 
confused about 2 components.

In this schematic:
R3 and C18 (I think it's 18, it's the .1uF cap in parallel with R2) are 
still confusing to me.

R3 seems to tie the base of the signal transistor weakly low, and it's 
been eliminated from the BN/E schematic.  As noted in our previous 
discussions (noted below) Gerritt suggests that it was due to the 
optimization, but I don't understand how the fact that the system uses 
the SuperPLA has any bearing on the presence or absence of the resistor:

On 2/17/2012 1:56 PM, Gerrit Heitsch wrote:
> On 02/17/2012 07:32 PM, Jim Brain wrote:
>> I was using this schematic:
> That's the schematic for the last rev. of the C64 board, the one with 
> the 64pin Super-PLA. There they had optimised about everything that 
> could be, down to eliminating the 556 for reset timing, using a gate 
> from a 74LS14 instead and integrating the 2114 color RAM into the 
> 64pin IC (250469 Rev. B).
>> While others used the TIP29 without a darlington arrangement:
> That's the schematics for a 250425, using the 8701 clock generator.
So, why would the 8701 clock generator and/or the SuperPLA negate the 
need for those two components?

Trying to figure it out (they are both gone from +4 and C128 schematics 
as well), I found that WRITE sprouted a 100 ohm resistor on the C16 and 

but, looks to be absent from the +4:

And then reappears on the C128:

I *assume* it's a current limiter in case someone accidentally shorts 
out the WRT line while it is being driven, but I could be wrong.


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Received on 2013-06-25 08:00:41

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