34s flashing cable possible candidate?

[cruff]

(06-24-2014 08:08 AM)jebem Wrote:  The serial interface works at TTL level, and the signals should swing betwwen 0Volt and 3.3Volt.

You need to make sure to test the voltage levels, as there are also 5V and 1.8V TTL serial interfaces in use also. An older cable may be more likely to be a 5V version, which would damage anything using a lower voltage I/O.