there is no on-board memory in those. 

> Can someone point me to real usful information about the DECstation3100
> (PM20A) on the web, or answer a few simple question (LED error codes, and
> the _real_ meaning, i.e. when it says "memory error" (no modules in the
> memory sockets), is it a fatal error, isn't there any on-board memory),
> etc, what cable to use with what monitor)
> 
> Sorry for this off-topic posting...
> 	Hamster (newbie to PMAXen)

hehe yeah, its also not a VAX :)
port-pmax would be a better place for this data... but anyhow, theres no 
place i know of that concentrates PMAX info anywhere!
(alas, there isnt much info on these wonderful machines available!) 
send me a private email if you have some specific questions, though, i
can probably help. i use these things constantly and also have some docs
if its obscure. There were once upon a time a few postscript files somewhere
deep in gatekeeper's bowels that described the PMX and the 3MAX internals
but not in detail for writing an operating system or anything, and also 
there were a few for some of the common TC peripherals. If you cant find 
them, ill giv you a guest account on guildvax.guild.net and you can ftp 
them from there..

happy hacking,
isildur