Curious to know what Class2 Serial chip you'll be using?

The H2 employs uses a variety of devices, for example the DDM (Drivers window switch) uses an "MC33990 Enhanced Class B Serial Transceiver" and "MC68HC908AS60AVFU 8-Bit Microcontroller" combination, whilst the Electronic Air Suspension Module uses an "MC68HC58 Data Link Controller" being a dedicated chip with complex functions/functions. One supposes GM selected these for a reason.

Not sure an ELM327 Chip can handle all the necessary protocols and traffic, though it's fine for OBD2 diagnostic adapters., it would be tempting to use the same device as the cluster.

If I can I'll try and get my hands on a cluster to reverse engineer.

I know the odometer chip can be reprogrammed to change the mileage in one. There are aftermarket tools available that most of the cluster repair places use for this. Generally speaking, if you have the correct board logic and eeprom in place that tracks and increments the mileage correctly, then it would only need to be programmed to the correct mileage for the vehicle it will be installed in. If for example you order a cluster from ISS or White then you simply provide them the current VIN and mileage - they program it in before they ship you the clusters.

On a side note - (Chris) White Automotive probably knows more about most of this than anyone. He is in fact the only one I know of who can custom program a GM BCM.

i know on my 03 cluster it uses an ATMEL eeprom
maybe the data can be padded and just leave what is necessary for the network to read it might do a CRC checksum of the data hence the padding needed you would have to watch the handshake procedure

I can do the same thing programming the odometer information at time of display shipment. It would have to be an overnight shipment and the person receiving the display would need the day off work to receive and install it. That way they don't drive around waiting for the display to arrive and create a change in odometer reading. Many ways to tackle this situation just trying to minimize any issues as I can.

ELM327 chip and devices are good for hobbyists. However, for production displays (Clusters) it's not what I'll be using. I'm creating my own boards using automotive grade components. Display will be a plug and play. My hardware will not be connecting to an OBD2 port. I'll show more of the development through video soon.

The OBD2 Port and Class2 Serial bus are one and the same in the earlier Hummers, however it is separated for CAN-Bus models 2008/9 according to the Service Manual, the IPC remains low-speed throughout.

The serial bus can be a source of consternation, one bad node can have dramatic effect.

There must be a "h/ a /c /k" for the odometer function one would think...

here is a few pics of the clusters i used the black face one is from a 2003 taho the white square one is the H2 cluster the board was after the eeprom swap showing the miles the h2 had at the time when the odometer screen failed.
https://cimg6.ibsrv.net/gimg/www.hum...72efd18ecc.jpg
taho donor with the taho miles
https://cimg8.ibsrv.net/gimg/www.hum...a3499fe041.jpg
H2 cluster with failed odometer
https://cimg7.ibsrv.net/gimg/www.hum...f515393db4.jpg
H2 and chevy cluster side by side
https://cimg9.ibsrv.net/gimg/www.hum...114cd875c1.jpg
taho donor board after eeprom swap
https://cimg0.ibsrv.net/gimg/www.hum...eeafe0bb94.jpg
atmel eeprom

I found this video on Youtube shows the eeprom chip. Jack has experience with programming and fixing the clusters. I reached out to him for more information. I'm surprised GM did not have a recall on these.

Only time pretty much any auto manufacturer is going to do a recall is if they are forced to (they will still call it voluntary though lol) due to a safety issue.

So its a 2K x 8Bit SPI EEPROM, it could be important to map out the contents to get an insight as to what parameters the ECU/BCM is expecting.

Mileage will most probably be encrypted with security checksums, but comparing EEPROM image snapshots after changing parameters, even mileage, it might be possible to map out some of the contents.