[Coco] Telepak RS232 Pak

[Gene Heskett]

On Saturday 17 January 2009, Roger Taylor wrote:
>At 03:53 AM 1/17/2009, you wrote:
>>On Saturday 17 January 2009, Roger Taylor wrote:
>> >At 10:31 PM 1/16/2009, you wrote:
>> >>Hi Mark,
>> >>
>> >>Thanks for the explanation. So if I understand correctly, the TelPak
>> >> uses a newer hardware technology and can be used without the services
>> >> of an MPI. Other than that they both do about the same thing...
>> >> correct?
>> >>
>> >> > I should expand.
>> >> >
>> >> > There is a MAX chip in the TelePak that has acharge pump in it that
>> >> > creates the RSR232 rail voltages.
>> >> >
>> >> > What this means is that you DO NOT need a multi-pak for this device
>> >> > to work in a CoCo3 as the Tandy pack does.
>> >
>> >I've never known the Deluxe RS-232 Pak to not work in a CoCo 3
>> >without an MPI.  There's a ROM in the pak which means you probably
>> >can't use it at the same time a floppy controller is plugged into Any
>> >CoCo unless you're using an MPI.
>>
>>It works if that teeny little Aztec 5 volt to +-12 volt switcher in it
>> works, but the draw on the 5 volt line is more than the cc3 supply can do
>> well.  I have seen several cc3s with blown fuses because that supply
>> hiccuped.  The one in mine let out all the smoke and died a decade ago, so
>> it works in an MPI only now.
>
>Why would the RS-232 pak need 12 volts?  The chips need 5v and are
>few, plus the DC Modem Pak has more circuitry with the phone modem
>portion so does it work on a CoCo 3? 

Because the signaling standards for RS-232 call for plus and minus 12 volts to 
be the swing level.  Therefore, they used a set of 1488-1489 level translator 
chips between the 6551 acia (or any other uart for that matter) and the 
output connector.

>The reason I'm asking this is 
>because we're working on a simple RS-232 pak the size of a game pak
>and I see nothing in it that should draw too much current from the
>CoCo.  My hacked DC Modem pak works well on my CoCo 3 and someone
>added the signal level converter ICs.

And works without the plus and minus 12 volts when plugged into a coco3 
directly?  That has to be an extremely well balanced circuit, it should not 
normally.  Not to mention that since you don't have a disk system then, 
writing a driver for it in basic would be quite a coding exercise.

So does mine, I made one of them out of a modem pack too, but it only works in 
an MPI because those level translator chips need its plus and minus 12 volts, 
which the coco 3 does not supply, and which the older coco's only supplied 
very marginally, around +-9 volts IIRC.  That is enough to make it work 99.9% 
of the time, but leaves precious little over drive for noise margins and poor 
or long cabling.

For all new designs, investigate the MAX-232 line of chips for that function, 
which contain their own charge pump level translators that are probably 500% 
more efficient than the Aztec mini-switcher in the RS-232 Deluxe pack ever 
thought of being.  That puppy ran HOT!  Or did in the 2 I have.  Neither have 
survived the ravages of age and 24/7 uptimes at the Heskett ranchette.

-- 
Cheers, Gene
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