Hello,

First, about me. I am quite a beginner regarding microcontrollers.  My 
profession is SAP application developer. As a hobby, I am learning 
embedded linux on a few project boards (gnublin), thereby learning linux 
and C.  I also own an Arduino Duemilanove (ATMEGA328) for educational 
purposes, an ATMEL "Butterfly" (ATMEGA169), also for educational 
purposes.
I have ordered the new raspberry pi, but am still waiting for delivery.
I have installed Atmel Studio 6 on my Win7 Laptop and am currently using 
it with the built-in device simulator to run assembler test code from a 
text book on programming tactics for AVR-microcontrollers.  My first 
embedded linux project is completed (home automation low cost web server 
- turning household appliances on and off via WWW).  My second embedded 
linux project is currently in progress (measure power generated by my 6 
KW-peak photovoltaic array and upload the data to my laptop Excel 
spreadsheet automatically at regular intervals via wlan).

My problem: the company which produced my photovoltaic array has gone 
bankrupt.  A feature of the array includes a datalogger which measures 
the generated power and reports it via cell-phone to a web site.  Since 
the company is bankrupt, the datalogger has become useless.  It consists 
primarily of a board with an ATMEGA128 16AU 0820 chip from Atmel, a 
cell-phone chip card, and a GE864-QUAD GPRS Module from Telit, and has a 
6-pin ISP connector.

My intention: I remember, years ago while learning x86 assembler I could 
reverse engineer Windows binaries with tools like WinICE and W32dasm. I 
would like to download the binary from the ATMEGA128, then reverse 
engineer it, then rewrite/modify it for my own use. Since I have not 
found any binary extraction facitity in Atmel Studio 6, I have installed 
WinAVR/AVRDUDE/AVRDUDE-Gui on my WinXP desktop, mounted the datalogger 
to a breadboard and wired up an RS-232 interface ( 
http://elm-chan.org/works/avrx/avrx_com.png , but TDX0/RDX0 instead of 
MISO/MOSI)to the ISP connector. When trying to download flash or EEPROM 
binaries from the ATMEGA128, I get errors like "avrdude: 
stk500_getsync(): not in sync: resp=0x00" "avrdude: stk500_disable(): 
protocol error, expect=0x14, resp=0x51" when I connect with programmer 
type "avrisp" and much less usefully errors when connecting with other 
programmer types.

My question:

Is my intention reasonably feasible, or am I just wasting my time? In 
other words, is it worth bothering to find out what I am doing wrong 
while connecting to the ATMEGA128, or is it simply not reasonably 
possible to reverse engineer as intended ( I have no CIA resources to 
help me)?

I have not yet tried flashing the chip, since I don't want to destroy 
the current programming which still appears to work, because I recognize 
the LED sequences which flash when board powers up ( I had watched the 
service guy when he originally installed the unit and set it up for 
hourly transmisstions).

Thanks in advance for expert advice.

Kind regards,
mgleep

Hey, thanks for the overwhelming number of constructive and helpfull 
responses!

In the meantime, I have built the Aquaticus AVR JTAG interface onto the 
breadboard where I already had the target board connected, identified 
the JTAG pins of the ATMega128 in the target board, and have connected 
them to the AVR JTAG interface.

Using the JTAG interface, I have been able to successfully connect to 
the target board and have downloaded the FLASH and EEPROM data. I am 
currently working on decompiling the hex data.

Great help! Thanks.
luser