Another catch-up post.
A few weeks ago, I accepted my first order for a T43 backlight kit. It turned out to be a c-c-c-c-c-combo-breaker!
In the early days of software-controlled brightness, ThinkPads used an analog brightness signal like just about every notebook. It was generated by one of the D-to-A pins on the embedded Renesas H8S microntroller all ThinkPads used.
As of the X40, ThinkPads went to using a digital PWM brightness control generated by the Intel Centrino ICH Southbridge. This made them kind of weird by laptop standards. It's one of the reasons I had to cons up custom LED drivers for my ThinkPad brightness kits.
In general, the T and X series of the same generation shared a basic architecture. The planars were quite different, but the chipset and basic design were the same. Not so with the T4X and X4X.
The X40-series is a completely different design from the T40-series. It uses a PWM brightness control. The T40, however, is analog like the older machines, which threw me for a loop at first. The good news is that I'd made working drivers for the X2x, X3x, T2x and T3x beforehand, so once I realized the T4x was an 'old' style, getting it to work wasn't hard. I can use the same positive-analog TLD2 hack that worked on the earlier models.
The bad news is all my fabrication, based on the TLD3, is geared toward the PWM-based ThinkPads. The TLD3 boards aren't able to use a positive-analog brightness signal no matter the hack. For a TLD3, it's PWM input or nothing.
I have on hand ~ 1500 TLD3 PCBs, waiting to be populated, for the usual PWM kits. I have only ~ 20 TLD2 PCBs left that can be pressed into analog use.
Get 'em while you can.