I recently got the bug to hack on my /240.  There is not much technical data
available on the DECstation hardware.

I have spent some time pouring over white papers on the R3000 and R4400
chips and though I've learned much about the MIPS chips, I now have more
questions than ever!!!

1) On the /240 CPU daughter card, there are 3 major chips.
    a) The heat sinked chip seems to be the R3000 CPU-FPU.
    b) What is the LSI L1A7242 and L1A7231?  LSI was recently bought by
Philips and there is
    little information on LSI chips.

2) The R3000 is a time multiplex 32-bit bus chip, the PGA R4400 was packaged
as 64bit bus, so is the bus going to the main board a 32 or 64bit path?

4) There is no reference to 144pin PGA R3000 packaged CPU anywhere, anyone
know exactly what make and model of R3000 DEC used?

3) Does the SG-615P chip, on the daughter card, control the CPU frequency?
If so, does it also generate the clock for the main board?

Reverse engineering is a slow process and I've never seen a /260 so if
anyone has schematics or other info they could copy and kick my way, I'd
love it.  The last time I was in this mode, I put together a Z8000 board,
but this is a bit more involved than that project and am still unsure of my
future success...

My Goal?  To either make a replacement board or devise a method to upgrade
an existing /2x0 to a bit more speedy technology.
So far the IDT9R3081 (http://128.11.21.63/docs/79R3081_DS_3483.pdf) looks as
if it may be a good CPU replacement with the help of a MQUAD/PLCC(84) to
PGA(144) adaptor (most likely a custom job) This would allow the board to be
ramped up to 50Mhz and get a bigger I/D cache.  However, the 50Mhz version
is a 5.0V chip, so I don't know if this available to the board without a
little wire hack :-)

As for the R4400 based /260, all I know is the R4600(133Mhz)chip is pin
compatible with the R4400 but has some issue that can be address with some
pin modifications per MIPS webpage
http://www.mips.com/Documentation/R4x00_common_design.pdf IDT make a R4400
based chip that may be a candidate for a retrofit
http://128.11.21.63/docs/79RC64474_DS_53993.pdf

I know this isn't really the forum for such a discussion, so if someone
insists that I take this offline, no problem.

jam