[Repairs] [support-gang] Fwd: Serial adapter for repair centers cost question

Richard A. Smith richard at laptop.org
Fri May 23 16:27:50 EDT 2008

John Gunkel wrote:

> The $0.50 cables would be ideal. Stick two taped between some
> cardstock in an envelope and send it out to all the repair centres.
> Those with the appropriate USB or serial converter will be immediately
> functional.For the rest, see below, or, a null modem cable (XO->XO)
> could be made at "the big end" of the cables by connecting both pins 4
> together, connecting one pin 2 to the other pin 3, and finally
> connecting the other pin 2 to the first's pin 3.

Right now I can't endorse XO<->XO at the TTL level.  Those pins go 
directly to the southbrige and I have no data on how robust they are. 
Until someone tests what happens with the various combinations of screw 
ups I want a interface chip in between.

Perhaps thats a good use of some of the boards that have other hardware 
problems.  Intentionally screw up your connections and see what happens.

>> Cautionary words, likely already said: These cables can be used to brick,
>> unbrick, and disable the security on XOs. If we're not prepared to deal with

Those cables alone cannot be used to unbrick or bypass security.  You 
also need a 65 Mhz clock that you either solder on or build into a 5 pin 

>>>> 1 is the cable that you use for connections to the laptop serial port.
>>>> Qty 500 I've got a quote for $.50.
> Now, If you have a quote for a cable, but have not yet ordered them,
> my suggestion would be to have the big end broken out as 3+1, that is,
> a female three pin connector on the one end for GND, TX and RX, then a
> single for the 3.3V signal. This makes the whole thing significantly
> more oops proof

Breaking it out may increase the cost since it probably adds a step but 
assuming XO<->XO direct connection is feasible might be a good idea. 
However, it adds complexity and makes it so I can't have the connector 
keyed unless I use a different connector.

> Picture this, a repair centre gets in a suspected brick, and you don't
> see anything on the screen. Is it a D6 brick, or did the "weekend
> developer" trash their x config trying to get a USB adapter working?
> (or any number of other possibilities) With a serial console, you can
> see at what level that problem is.

The onboard OpenFirmware diagnostics already answer most of these 
questions.  Only a hardware problem or firmware problem can prevent the 
screen from starting.  Diagnosing that at a deeper level would require a 
serial port but you need developer level knowledge.

If the onboard firmware diagnostics can be run successfully then there a 
high degree of confidence that that hardware is ok.

Richard Smith  <richard at laptop.org>
One Laptop Per Child

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