Just a quick update:

I used the far more simplified circuit, noted here:

Make a datalogger cable | Galant VR-4 > How To and Info Archive | GalantVR-4.org Mitsubishi Galant VR4 Forum
The Laptop version is what I used, not the palm version.

Then, took the RS232 and converted to TTL using the MAX232 (refer to the datasheet, it's very easy).

Then, connected to the arduino, then used the information on MMCD's page (1920 baud, 0x21 for RPM).

RPM values were very consistent and correct.

This tells me that for DSM / Mitsubishi 4G63 / Hyundai G4CP engines, that ALDL works correctly when the circuit for the interface is done correctly.

I can now proceed and take the request / responses and try and push that into the MPGuino code.

I'm waiting on an LCD display to start putting it together, but will start hacking out the code soon.

I can't get reliable readings from speed though!

I queried speed using the value noted online in a few places, and seemingly used by Evoscan (another Mitsubishi data logging / tune up tool).

The responses for speed varied wildly, see below:

Speed: Result: Dat 0: 2FDat 1: 00|Speed: 0

Speed: Result: Dat 0: 2FDat 1: 7777|Speed: 238

Speed: Result: Dat 0: 2FDat 1: D6D6|Speed: 428

Speed: Result: Dat 0: 2FDat 1: 9F9F|Speed: 318

Speed: Result: Dat 0: 2FDat 1: 2B2B|Speed: 86

Speed: Result: Dat 0: 2FDat 1: 7C7C|Speed: 248

Speed: Result: Dat 0: 2FDat 1: DDDD|Speed: 442

Speed: Result: Dat 0: 2FDat 1: ADAD|Speed: 346

Speed: Result: Dat 0: 2FDat 1: CCCC|Speed: 408

Speed: Result: Dat 0: 2FDat 1: 3333|Speed: 102

Speed: Result: Dat 0: 2FDat 1: 1313|Speed: 38

Speed: Result: Dat 0: 2FDat 1: 3232|Speed: 100

Speed: Result: Dat 0: 2FDat 1: CECE|Speed: 412

Speed: Result: Dat 0: 2FDat 1: C8C8|Speed: 400

Speed: Result: Dat 0: 2FDat 1: E6E6|Speed: 460

Speed: Result: Dat 0: 2FDat 1: A8A8|Speed: 336

Speed: Result: Dat 0: 2FDat 1: C3C3|Speed: 390

Speed: Result: Dat 0: 2FDat 1: 2C2C|Speed: 88

Speed: Result: Dat 0: 2FDat 1: 3737|Speed: 110

Speed: Result: Dat 0: 2FDat 1: 5A5A|Speed: 180

Speed: Result: Dat 0: 2FDat 1: 2929|Speed: 82

Speed: Result: Dat 0: 2FDat 1: 2D2D|Speed: 90

Speed: Result: Dat 0: 2FDat 1: 3636|Speed: 108

Speed: Result: Dat 0: 2FDat 1: 3C3C|Speed: 120

Speed: Result: Dat 0: 2FDat 1: 6C6C|Speed: 216

Speed: Result: Dat 0: 2FDat 1: 1E1E|Speed: 60

Speed: Result: Dat 0: 2FDat 1: 7C7C|Speed: 248

Speed: Result: Dat 0: 2FDat 1: 2222|Speed: 68

Speed: Result: Dat 0: 2FDat 1: 1111|Speed: 34

Speed: Result: Dat 0: 2FDat 1: 4B4B|Speed: 150

Speed: Result: Dat 0: 2FDat 1: 8989|Speed: 274

Speed: Result: Dat 0: 2FDat 1: 8686|Speed: 268

Speed: Result: Dat 0: 2FDat 1: 44|Speed: 8

Speed: Result: Dat 0: 2FDat 1: 1B1B|Speed: 54

Speed: Result: Dat 0: 2FDat 1: 66|Speed: 12

Speed: Result: Dat 0: 2FDat 1: 2424|Speed: 72

Speed: Result: Dat 0: 2FDat 1: BABA|Speed: 372

Speed: Result: Dat 0: 2FDat 1: 33|Speed: 6

Speed: Result: Dat 0: 2FDat 1: DEDE|Speed: 444

Speed: Result: Dat 0: 2FDat 1: 8686|Speed: 268

Speed: Result: Dat 0: 2FDat 1: E0E0|Speed: 448

Speed: Result: Dat 0: 2FDat 1: 88|Speed: 16

Speed: Result: Dat 0: 2FDat 1: B1B1|Speed: 354

Speed: Result: Dat 0: 2FDat 1: 2B2B|Speed: 86

Speed: Result: Dat 0: 2FDat 1: 2828|Speed: 80

Speed: Result: Dat 0: 2FDat 1: 9090|Speed: 288

Speed: Result: Dat 0: 2FDat 1: F3F3|Speed: 486

Speed: Result: Dat 0: 2FDat 1: DD|Speed: 26

Speed: Result: Dat 0: 2FDat 1: 2B2B|Speed: 86

Speed: Result: Dat 0: 2FDat 1: EDED|Speed: 474

Speed: Result: Dat 0: 2FDat 1: 5454|Speed: 168

Speed: Result: Dat 0: 2FDat 1: C3C3|Speed: 390

Speed: Result: Dat 0: 2FDat 1: 3C3C|Speed: 120

Speed: Result: Dat 0: 2FDat 1: E0E0|Speed: 448

Speed: Result: Dat 0: 2FDat 1: 6767|Speed: 206

Speed: Result: Dat 0: 2FDat 1: 9F9F|Speed: 318

Speed: Result: Dat 0: 2FDat 1: B4B4|Speed: 360

Speed: Result: Dat 0: 2FDat 1: D6D6|Speed: 428

Speed: Result: Dat 0: 2FDat 1: 1F1F|Speed: 62

Speed: Result: Dat 0: 2FDat 1: 3D3D|Speed: 122

Speed: Result: Dat 0: 2FDat 1: F9F9|Speed: 498

Speed: Result: Dat 0: 2FDat 1: 1E1E|Speed: 60

- My little car couldn't ever achieve speeds of 498km/hr, and if it did, I certainly wasn't being an eco driver - maybe the calculation is wrong.

This gives me solid confirmation that the circuit works tho, this is great.

If it wasn't the circuit that fixed it, it would have been the 10ms delay after the data was requested.