Extending the life expectancy of a modchipped v1.6 xbox
In a v1.6 xbox, one of the data signals on the motherboard (LFRAME) is short circuited to ground to let a modchip boot. This puts a strain on the xbox electronics whenever the modchip is operating. Although the modchips work, the life expectancy of the xbox may be shortened by doing this - much like driving your car around with the engine always running at 6000rpm (the car still goes, but the engine will get warn out much more quickly). A resistor can be used to prevent the problem of installing a modchip, and extend the life of the xbox.
A more detailed description:
The bios on a v1.6 xbox is loaded from the LPC bus, the same bus that the modchip uses. Therefore there will be two different bios (modchip and normal MS bios) both trying to load at the same time onto the same wires. v1.0-1.5 xboxs used separate wires to load the MS bios compared to the modchip and so this wasn’t a problem.
So how do we stop the MS bios from loading onto the wires we want to use for the modchip in a v1.6?
LFRAME is used to signal the start/end of a data transfer on the LPC bus. In v1.3-1.5 xboxs MS removed this signal from the motherboard to make "cheapmod" modchips no longer work - so modchip makers generated this missing signal with some extra electronics on each modchip. On v1.6 xboxs we short out the LFRAME signal on the motherboard so that the bios chip on the motherboard doesn't know it is being sent data (it can't see the start signal) and so it doesn't do anything. The modchips now generate the missing signal themselves, so they see the data correctly and can load up without any problem.
So what is the effect of shorting out LFRAME?
Shorting out a signal causes a higher current to flow than is normal. In v1.0-1.5 xboxs we short out D0. This causes a current to flow during the time the onboard bios is trying to load - which is a maximum of a few hundredths of a second. After this it is disabled so no current flows, and the modchip loads up.
On the v1.6 we are shorting a different signal, LFRAME, which as far as the xbox is concerned is required to load the bios in any circumstance. This causes a problem - the signal does not turn off when the modchip loads like the v1.0-1.5 xboxs do. We have a continuous current flowing whenever the modchip is being used - a current which is coming from a data line driver which is not designed to do this!
For those interested, I measured 62mA and 68mA on two different 1.6 xboxs - to put this into perspective a standard 3.3V 74LV series IC output normally operates at a few mA, and has an absolute maximum rating of 25mA (I am comparing with other generic electronic components as I obviously don't have the actual xbox component datasheets - anyone who is feeling generous feel free to send me a copy :-) Some new modchips, such as the Aladdin XT, have had to add an extra point ‘X’ to try and deal with this high current as the original Aladdin modchips cannot cope.
One fix for this is quite simple, cut the data track on the motherboard and insert a resistor. With the modchip off, the resistor has very little effect on the data line and so the xbox boots normally. With the modchip on, the resistor limits the current to less than 5mA (less than one-tenth of the original value) and so prevents any long term damage occuring to the xbox.
If I have made any errors in this description let me know and I will edit it appropriately.
UPDATE: Modchips which pulse LFRAME and therefore do not suffer from this problem:
(These results are from posts in this thread - I have not confirmed them)
Edited by catdog2, 23 November 2004 - 08:48 PM.