[Tfug] Interchangeability of VRM's?

[Adrian]

On Monday 10 December 2012 12:52, Bexley Hall wrote:

> My assumption is that VRM's are just "very local" voltage
> regulators designed to tweek the supply voltage to the
> processor on a processor-specific (or even "on demand")
> basis.

More or less, there are 4 (or was that 5?) digital lines that select the 
output voltage.


> But, how universal are the VRM's?  I assume *less* universal
> than "PC power supplies" (which don't care if you're running
> AMD, Intel or WhoopdeeDoDah).  I.e., are the signalling bits
> from the processor to the VRM somewhat standardized?

> Similarly, does the VRM connect to the host/motherboard by
> some standardized interface?  Or, is the VRM seen as an extension
> of the motherboard?

Excluding the manufacturer specific versions (Dell/HP/Compaq/etc. had variants 
specifically for their motherboards), VRMs were standardized, but not 
necessarily interchangeable.

As I vaguely recall... there were 2 formats of VRMs: one was a dual-inline pin 
(about 2x20 pins), the other was a slot connector, and hence not 
interchangeable between formats. There were also a number of different 
versions, in increasing revision number. Generally, the higher the revision 
number the greater the number of voltage output options (i.e. an early P3 may 
need a 8.1 spec, a later P3 needing a different voltage may need a 8.2 spec). 
Generally, revisions were backward compatible.

To make matters even more complicated, the VRM spec assigned 5V and 12V input 
rails on the connector, but neither was mandatory. So there were 5V input 
VRMs, 12V input VRMs, 5V/12V either-or "universal" VRMs (where the VRM would 
run off either rail if present), and 5V/12V required VRMs (where the VRM 
required both rails to operate). The motherboard manufactures also choose to 
not always implement both rails... so there were some motherboards with only 
5V or only 12V supplies to the VRM.

Therefore, you had to match the VRM to the type of motherboard supply you had. 
A 5V VRM would not work in a 12V-only board, or vice-versa, but a 
5V/12V "universal" would work in either motherboard. 

I did a writeup on differences between a bunch of different VXI-brand VRMs 
some 10 years back (when I was selling a bunch of refurbs), but I can;t seem 
to find it now. Intel had a number of very information design spec documents 
at the time. Search for the Intel 8.1, 8.2, 8.3, and 9.0 VRMs design pdfs.


Adrian