QUOTE(Madhatta @ Feb 28 2011, 05:48 PM)
Syntaxerror: I am by no means trying to step on what you said as you know your stuff way more than most people here but is that really the JTAG wiring for him to use? He said he is on a Xenon. I have read that for Xenons you don't want to use that method and instead use one like listed here (second picture down):
100% for sure there is nothing better then the transistor wiring for all motherboard versions. Your advice on properly remapping the bad blocks is very wise. That could very well be the issue.
The following information was written by BLACKADDR and is included with his Xbox 360 SMC Utility.
FYI. Blackaddr is a genious at writing code for SMC and he has incredible understanding of circuits.
The diode method provides a safe electrical circuit by disconnecting (through high impedance) the SMC when it drives 3.3V, but allowing the SMC to pull the signal to 0V when driving logic zero. There will be a voltage drop across the diode when SMC drives zero, so the JTAG still sees a positive voltage. If this voltage is > (0.35 * 1.8) = 0.63 V, the signal will not be detected properly as a logic ‘0’. In order for the diode to be reliable, it should have a low forward voltage. Schottky diodes (such as BAT41) are specifically designed to be fast switching with low forward voltages (< 0.5V) and are very reliable for JTAG. Small-signal or general purpose diodes on the other hand (such as 1N914 and 1N4148) have forward voltages of ~ 0.60V for the current in this circuit. Many users have experienced unreliable booting or slow booting with the use of non-Schottky diodes but trying them will not cause damage if Schottky’s aren’t available.The console will retry rebooting up to 5 times if the exploit fails. Slow boots and random E79s can be the result of corrupt JTAG transactions caused by non-Schottky diodes.PROS: Very few components required (2x diodes). Electrically safe level translation. Any Schottky diode should work reliably, any general purpose diode is safe but might be less reliable.CONS: Schottky diodes are less common than general purpose diodes and harder to find. If BAT41 cannot be found, it may be difficult to find a substitute if you are not familiar with other Schottky part numbers.
vs TRANSISTOR METHOD
The transistor method provides a safe electrical circuit for level translation by disconnecting the SMC (through high impedance) when it drives 3.3V, but allowing the SMC to pull the signal to 0V when driving logic zero. There will be a very small voltage drop across the transistor, so the JTAG still sees a positive voltage, however it is typically < 0.1V, well below the 0.6V threshold. This ensures both logic 1's and logic 0's are correctly captured. Transistors must be NPN for this circuit. Pinout for 2N3904 is shown below, different transistors may have different pinouts so check their datasheet.PROS: Reliable, electrically safe GPU JTAG transactions. Uses very common, easy to find, easy to substitute components. Any NPN transistor can be used (double check pinout). Any resistor in the 10K to 20K range should work.CONS: More components and soldering. (2x NPN transistor, 2x resistor).
So basically the transistor method is the most electrically safe and provides the highest quality signal possible to the GPU. This reduces that chances of ever having a failed boot. I have NEVER has a jtag fail to boot since using this method. (besides the ones that got RROD but thats not the jtag's fault)