I am back from vacation, good to see VMS discussion is still going strong!
FYI, bus control (or signal processing or A/D ... all names are synonymous with the same component) chip replacement is the second most common repair I do. In my article "Brains, Batteries, and Busses"
http://eaaforums.org/showthread.php?...ies-amp-Busses
part of what I discuss is the analog board and the bus chips. The bus chips are the first processing component in the DPU to encounter the raw analog signal coming from the various sensors and transducers. As such, they are the most suseptable to voltage spike damage (static discharge, starter kickback, intermittant shorts, etc). It is possible for only one or a couple of channels on the bus to be damaged, in this case for example you may have one cylinder where the EGT graph is blank but all others work fine.
When replacing these chips, its important to match the original component as closely as possible. In the case of the bus chips, older (screw terminal type) DPUs use a different chip than the later serial type DPUs. As one poster from above noted, in the case of the serial DPU, exact replacements may not be readily available (I maintain a stock of the correct chips) from the common electronics shops like Digikey or Mouser. For this case it was critically important to get a chip rated for the same or lower amperage draw which the individual did.
The other recommendation I have is to make sure you have the right tools and equipment to do the job. i have lost count of the number of repairs I have done for folks who tried to DITY and damaged their DPU worse either from static discharge (they did not have a static free workstation and/or failed to take static precautions) or from using a hardware store soldering iron on an IC and ruining it and/or burning up the copper etching on the PC board.
Quick note also on the fuel flow accuracy issue ... FYI my EPI-800 totalizer is typically within 0.1 - 0.3 gal of actual fuel at fillup, it has never been more than 0.7 gal off so the EPI-800 and VM1000 systems are fully capable of very accurate fuel computing based on flow. Errors come into play from 3 sources 1) air bubbles or excessive turbulence in the fuel system 2) K factor variations 3) Power or signal interruptions.
Here I will address K factor variations ... for carb engines, FloScan manufactured the model 264 with an integral pulse damper in the unit with a K factor approx 47k (I dont have the tech specs in front of me so I am typing from feeble memory) ... Production ceased on this unit and the replacement is the 201B-9 with an external pulse damper and K factor approx 44.7k ... So you do the math and see that from the get go you have a 4.9% error when you replace a 264 with a new 201B-9. The newer serial type DPUs with software rev 2.0 and higher have a K factor adjustment feature, older DPUs do not and cannot be upgraded.
All the best,
Reggie