Did it. Grah.
Ignition Switch to "ON": 0000 Rx
Start Car: 00 (repeated until starter is turned off)
But that's it. I sent command after command (using the right settings for the port in HCT) and got nothing back.
EDIT: Did some digging on Google. I now suspect less that it is my USB device, as the device I have (F5U109) is a rebadged U232-P9, which was designed specifically to adhere exactly with Serial protocols and avoid the problems with USB-Serial devices. And doing a search on that came up with this thread for a GPS unit:
http://www.audiovisualtalk.com/showthread.php?t=25072
...which is EXACTLY what's going on here. I can receive ECU data just fine, as shown by the data received upon initial turnkey. But I cannot send any data at all. So I suspect there is a conflict or something on my laptop which is preventing proper serial operation. I will investigate further, but any help others can provide would be greatly appreciated.
EDIT2: And now I've found something else. I'm not sure how to implement it, as it's a fix for a Linux distribution... but the text was interesting:
Brief Patch Description:
Fix a problem in the 'mct_u232' driver whereby output data gets held up in the
USB/RS-232 adapter for RS-232 devices which don't assert the 'CTS' signal.
Problem:
Although _reads_ from either of the RS-232 devices mentioned above work fine
via the Belkin adapter, _writes_ to the GPS receiver are not being seen by
the GPS. Writes to the modem, however, work perfectly.
Investigation:
Using the 'seyon' terminal emulator in Linux and a crude hardware RS-232
"breakout box" that I hacked together, I've determined that the problem is
related to the RTS/CTS RS-232 hardware handshaking.
After further investigation, I've concluded that RS-232 devices which do not
assert the 'Clear To Send' ('CTS') signal prevent the Belkin F5U109 adapter
from transmitting data to the RS-232 device when the current (version 1.1)
'mct_u232' module is used. The data gets "queued up" (up to a point -- 16
bytes, I think) in the adapter but never transmitted.
=============
Now, he goes on to say that the Windows 98SE driver sends two signals to the Belkin to emulate the CTS. I'm using the official Belkin XP driver for the device, but I guess it's possible that it's different or something? Why is Serial communication to darn complex?