[time-nuts] (OT) HP 16500B / 16500L - reprogramming MAC address

Hal Murray hmurray at megapathdsl.net
Fri Feb 18 21:50:09 UTC 2011


>  From what I can gather, the (soldered down) 28C16 EEPROM on the 16500L  LAN
> card has either been wiped by an errant write, or has 'forgotten'  its
> contents.

My guess is something is broken rather than the chip has been wiped.  Have 
you checked the chip select or data path?

> Does anyone have a dump of the contents of this chip from a working  16500L
> card?

If it's the MAC address, it's unique to each board.  Do you remember the old 
one?  If not, you can either cheat and make one up, or find an old board, use 
its MAC address, and destroy that board so you can't possibly get a duplicate 
on the net when that board gets plugged in.

In the early Ethernet days, the MAC address was typically stored in a small 
ROM.  Since even the smallest ROM chips available had lots of room they added 
some other stuff.  A description of the format is probably available online 
and/or you can probably decode it from some driver software that reads the 
ROM.

A MAC address is 6 bytes.  I think they added 2 bytes of CRC.  A common small 
ROM was 32x8, so that would have room for 4 copies of an 8 byte MAC 
address+CRC.  I think they had the address, it's inverse, and a couple of 
test patterns.

As silicon marched on, typical TTL ROMs fell off the edge of the world and 
EEROMs became cheaper.  So 32 bytes of data was stored in chips that could 
hold xK bytes.


> Failing that, does anyone know of a way to reprogram the MAC address  into
> the chip? Some secret key-combination, or other trick?

I doubt if there is a simple way.  Why would you want to do that?  The idea 
is that each board should have a unique MAC address.  You want to make it 
hard for idiots to do things like copy an address and destroy that uniqueness.

The boards typically disabled the write-enable pin.  The chips were 
programmed before getting soldered down.  Or maybe they were programmed on 
the tester which would overdrive a pullup.


-- 
These are my opinions, not necessarily my employer's.  I hate spam.






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