[NTLK] TTL serial cable: are they useless?

web at kobier.id.au web at kobier.id.au
Sat May 11 21:34:21 EDT 2019

Hi Rufus,

The simple answer is the TTL cable is no good..

BUT the long answer is

The difference between the TTL cable and the RS232 cable is the voltage levels used.

TTL uses TTL voltage levels of 0V for 0 and 5V for 1
RS232 uses between +3 and+12V for 0 and -3 and -12V for 1

TTL cables are intended for direct connection to say a microcontroler but RS232 is intended for long distance communications.

To add to the confusion the Newton actual uses RS422 for the data comms which is a differential singling system which is basically double RS232 it has 2 transmit lines and 2 rx lines (one is the inverse of the other) to use rs232 you tie one to ground and use the other (to maintain the correct voltage polarity for the signal)
As you might have noticed RS232 (and422) have a wide valid voltage range 3 to 12V so it is possible to invert the TTL signal with a CMOS inverter and it usually works. The CMOS inverter is required because it can accept a possible 12V into it.

Sorry the more I type the more involved this is becoming :-)

Now depending on the ACTUAL voltage output by the Newton - it could be anywhere from 3 - 12V Ive never measured it but if it is no more than 5V it could be possible to use the +Ve data pins (6 and 7) and tie the -Ve data pins 3 and 5 to gnd (this is the opposite to what is required for RS232 ) and use the TTL cable. NOTE I have not tried this or made the appropriate measurements it is just theoretically possible. I would not just TRY it with out making measurements first or damage may result.



> On 12 May 2019, at 06:19, Rufus Kohn <rufuskohn at gmail.com> wrote:
> Hi
> I bought a TTL serial to usb cable and found out it wont work with the
> Newton. The mac recognizes the FDTI chipset but it is not possible to
> connect via NCX. So i bought the correct cable but it left me wondering
> exactly whet the difference is between the two.
> They look identical.
> Is the difference hardware? Like are the wires hooked up in a different
> way? Could I switch them around to make it work?
> Or is it a software difference and it will never work on a newton?
> Who can enlighten me?
> -- 
> Met hartelijke groet,
> Rufus Kohn
> 0613159031
> ----------------------------------------------------------------------
> http://newtontalk.net/
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