Ok, I spend some time guessing the 20 pin header, and I found the serial Tx and after a lot more time, the Rx. The Rx took so much longer, because if I connected it before powerup, the device would not in fact power up. Possibly the Rx is also a JTAG function, and the 3.3V idle of my cable's Tx triggers some JTAG function or something. So, I have to connect the power and then quickly connect the cable's Tx to the device Rx and press Esc. Or, if I was too slow, wait for a full VxWorks boot (about a minute), and then type the command reboot ...
This is what I found:
10 = RxD
14 = TxD
11 = Reset (or behaves so)
12,15,18,19,20 = GND
all others I wrote the measured average signal level
0V signal 1 2 3.3V signal
3.2V signal 3 4 3.3V signal
0V signal 5 6 3.3V signal
3.2V signal 7 8 3.3V signal
3.2V signal 9 10 RxD
Reset 11 12 GND
3.2V signal 13 14 TxD
GND 15 16 3.2V signal
0V signal 17 18 GND
GND 19 20 GND
The square pin is of course pin 1 and is the one closest to the crystal
So you can nicely connect the serial to the row of pins 10,12,14
Next to see if I can compile or find an AR7 image small enough to fit in 2Mb
If anyone has experiance with the PSPBoot or TI AR7, and can hint me at some does or donts or obvious or less obvious traps, feel free. Some relevant device data is shown in the SC101 data page.