[Coco] MPI problem or ...???

[Robert Gault]

Brian Blake wrote:

> Greetings all. I ran into a problem with my MPI this weekend.
 ><snip>
> Then, I tried another 512k CoCo 3. First, with just the FD-502 and
> everything works fine. Next, the MPI and FD-502 in slot 4, everything
> worked. Then the SS/C in slot 3, everything works fine. Next up was
> with the O-90 in slot 2. Not fine... I noticed immediately that it was
> booting to DECB 1.1 instead of 2.1. ><snip>

  This type of problem looks like a power supply defect in the MPI. Load 
up enough devices and the MPI flakes out.
  A good test for this is to change the order of paks and their 
positions in the MPI. For example, what happens with an MPI, a disk 
controller in slot1, the ORC-90 pak in slot4, and the MPI switch at 
slot1? If the Coco will power up in DOS 2.1, the problem is not with the 
ORC-90 pak. It will either be a non-modified MPI or power supply problem.
  A test of modified vs non-modified, is to start a Coco3 without an MPI 
and PEEK $FF90-$FF99, see what the values are, plug in the MPI and see 
if the values change. If the values change, the MPI most likely is not 
modified for Coco3 use.
  Power supply failure normally is a loose or oxidized contact on the 
main power transistor. This is caused by a screw making a bad contact 
with the transistor case. Loosen the screw/lock washer and re-tighten 
it. That should improve the situation.