[volt-nuts] Guide to programming the Hitachi 6303?

David C. Partridge david.partridge at perdrix.co.uk
Tue Apr 24 14:38:57 UTC 2012

I believe the problem is now identified thanks to an enormous amount of help from Paul Flinders who got me to understand the floating CPU startup code.

There is a watchdog timer which is supposed to reset the floating CPU if there is no data received for >0.2s, trouble is that the 74LS161 that handles this logic is as dead as a doorpost.

So instead of the intial 0x00 from the earthy side resulting in a RESET pulse (invert of RCO from the '161) which in turn results in a mains frequency identifier character (0x31, 0x32, or 0x33) being sent to the earthy side, the floating processor tried to process it as a command and clearly didn't quite understand it, so sent back a 0xc6 which the earthy side didn't understand - result "MAINS FAULT".

David Partridge
-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On Behalf Of David C. Partridge
Sent: 23 April 2012 21:08
To: 'Discussion of precise voltage measurement'; hp_agilent_equipment at yahoogroups.com; TekScopes at yahoogroups.com
Subject: [volt-nuts] Guide to programming the Hitachi 6303?

I'm trying to understand the floating ROM code for my 7081 (many thanks to Mickle T. for stating the reverse engineering job and pointing me to the right tools).

Why am I doing this?  I want to fix a broken 7081 which is reporting MAINS FAULT on power up.

I am massively hampered in this task as I don't have any experience of assembly programming 6800 or similar CPUs (or indeed any micro processors).  Are there any good resources online I can download, or books I can buy to that will get me on the road.

I know this request may be in vain as this stuff is now considered WAY out of date.

David Partridge

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