Apple //c won't boot when serial connected?!?!

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BadDadBad's picture
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Joined: Mar 25 2016
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Hi all,

Just acquired a NTSC //c ROM0 , and have an odd problem with the serial port (modem). If the //c is connected to my PC with null modem cable, and PC is powered on, the //c won't power on, nothing, no LEDs, remove the serial cable and it comes to life, reconnect the cable and I can send serial data to the //c from the PC, ADTPro won't receive a file list from the PC, I have a suspicion the //c won't send data to the PC.

I have tried using a Mac mini and USB serial adaptor with the same results. Incidentally both PC and Mac ADTPro work with my IIe, obviously a different cable.

Have checked my cable with my DMM and all seems fine.

Anyone have any thoughts where the problem may be?

Thanks for any input

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speedyG's picture
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Joined: Nov 16 2011
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Re: Apple //c won't boot when serial connected?!?!

Hello BadDadBad,

welcome to the community.

Did you only measure from pin at modem connector to serial at PC straight
or did you also check for possible shortcuts to neighbor pins and shielding ?
( specially at the tiny DIN8 plug for the IIc ? )

Did you follow instructions from ADT-site making that cable ?

sincerely
speedyG

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BadDadBad's picture
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Re: Apple //c won't boot when serial connected?!?!

Hi speedyG,

Yep, followed the ADTPro wiring for the //c, it's a 5 pin din on the machine rather than an 8pin mini din though. I will check the machine for shorts, only checked the cable for continuity and shorts and also the voltage for tx/rx to ground from the PC. I'm literally only carrying these three pins (2,3 and 4) into the din 5, pins 1 and 5 are shorted as ADTPro recommends.

Seems to me that supplying +10v to tx and rx is stopping the //c switching on.

Guess it's time to start some disassembly. Will post back when I (hopefully)find a solution.

Thanks for your help so far.

speedyG's picture
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Joined: Nov 16 2011
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Re: Apple //c won't boot when serial connected?!?!

Well at the moment without confirmation it´s just a suspection....
but the circuits are related to the grounding.....
if the IIc isn´t powered up and there is no grounding then the
resistors at NMI, Reset and IRQ are not in fixed relation to Ground
and may drift off expected values.....

the voltages at TX and RX are in fixed relation to the GND of the PC
if that unit is powered up ..... but if the Apple IIc is at that moment not powered up
and has no connection with Ground then strange things might happen
like drifting off the expexted values at the Apple IIc....

then after the IIc is powered up system allocates it´s own grounding
but reset may still stay a while at "drifted off state" preventing the system from startup...

I´d guess the mistake to disappear if there is a proper connection
between Ground of the PC and Ground of the Apple IIc.

speedyG

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gsmcten's picture
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Joined: Oct 4 2005
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Re: Apple //c won't boot when serial connected?!?!

BadDadBad,

Welcome!

Steven Smile

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BadDadBad's picture
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Re: Apple //c won't boot when serial connected?!?!

Thanks SpeedyG for your suggestion, going to try and dig more this weekend, will post results if I get any!

BadDadBad's picture
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Re: Apple //c won't boot when serial connected?!?!

gsmcten wrote:

BadDadBad,

Welcome!

Steven Smile

Thanks Smile