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: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/251138-2of2.gif 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: >> http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/250469-rev.A-left.gif >> > > 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: >> http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/251469-2of2.gif >> > > 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 C116: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/plus4/c16-251788-1of3-right.gif http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/plus4/c116-251239-1.gif but, looks to be absent from the +4: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/plus4/plus4-310164-1of4.gif And then reappears on the C128: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/310378-3-left.gif 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. Jim Message was sent through the cbm-hackers mailing listReceived on 2013-06-25 08:00:41
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