Author Topic: Rom options for HP 3456A -- Result!  (Read 1093 times)

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Offline essele

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Rom options for HP 3456A -- Result!
« on: August 30, 2018, 11:34:56 am »
Hi,

I'm working on an HP 3456A (for fun and the learning experience) and I've determined that one of the rom chips is faulty.  I've managed to build myself a small adapter board so that I can read them on a TL866, and two of them match the checksum details on the info you can find from Stepan Novotill, the other one doesn't, and in fact the checksum seems to change each time I read it!

I'll solder sockets on the board, but I'm also thinking about making a small adapter board to allow me to use the PLCC32 variant of the AT28C64 as this is socketable to allow easy programming and also will fit nicely within the DIP24 footprint.

Is there any reason (other than my ability :-)) why this wouldn't work?

Then as a follow-on question ... what do people use for the pins when making a board designed to fit into an IC socket? ... normal headers seem way to big, especially for the round pin sockets, I've found some round pin headers on eBay and Aliexpress, but I'm surprised I can't find anything suitable at Farnell or Mouser etc??

I know I could solder the adapter right onto the board using normal headers, but I'd rather put standard sockets on there.

Cheers,

Lee.
« Last Edit: September 07, 2018, 06:18:21 pm by essele »
 


Offline essele

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Re: Rom options for HP 3456A
« Reply #2 on: August 30, 2018, 12:26:17 pm »
Thanks oPossum -- I'll stick with the cheap Chinese versions!
 

Offline JFJ

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Re: Rom options for HP 3456A
« Reply #3 on: August 30, 2018, 02:24:27 pm »
... I've determined that one of the rom chips is faulty...

Is the faulty chip an actual Mostek MK36000 series ROM or a windowed EPROM?

EPROMs exist that are pin compatible with the Mostek ROM, e.g:


They are still available via eBay:
https://www.ebay.com/sch/i.html?_from=R40&_nkw=MCM68766&_sop=15

However, if the firmware is already in a UV EPROM, then beware that a static charge on the window can corrupt data reads (hence, the varying checksum). This can be easily remedied by giving the EPROM a quick burst in a UV eraser (say 30 seconds), to dissipate the static charge.
 

Offline JFJ

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Re: Rom options for HP 3456A
« Reply #4 on: August 30, 2018, 03:04:29 pm »
... I've found some round pin headers on eBay and Aliexpress, but I'm surprised I can't find anything suitable at Farnell or Mouser etc??

RS Components still do them:
https://uk.rs-online.com/web/c/connectors/pcb-connectors/pcb-headers/?searchTerm=Preci-Dip%20Header%20Pin
 

Offline MadTux

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Re: Rom options for HP 3456A
« Reply #5 on: August 30, 2018, 04:33:36 pm »
IMO it's much simpler to hack a DIP-28 socket into the best suitable DIP-24 footprint (U3) and botchwire the rest so that a 27C256 with FF padding on unused ROM locations can be used (can be found online here). The excess 4 pins on DIP-28 socket are left floating in space and are air-wired with isolated copper wire. The wiring was then secured with clear universal glue. Totally adequate for the purpose and much less work than to build/solder adapter board.

BTW, be careful with new EEPROM/Flash memory. I found new memory models are too fast for old 6800 processor so you get timing problems i.e. it won't work.

Pics of hacked board, note the cut traces on the bottom.
« Last Edit: August 30, 2018, 04:44:42 pm by MadTux »
 

Offline essele

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Re: Rom options for HP 3456A
« Reply #6 on: August 30, 2018, 05:15:03 pm »
Hi JFJ,

The failed device is the original MOSTEK MKxxxx ROM, I know there are some available from eBay, but by the time I've paid shipping I may as well have had the fun trying to get an adapter built.

Cheers,

Lee.
 

Offline essele

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Re: Rom options for HP 3456A
« Reply #7 on: August 30, 2018, 05:22:14 pm »
Hi MadTux,

Thanks for the info and pics, very useful ... although if you had seen my 'botchwire' adapter for the TL866 you might reconsider the "much simpler" comment :-)

I've just ordered the board ... hopefully it will be a good learning experience, and they are so cheap now I thought it was worth a punt.

I will let you know how it goes.

Cheers,

Lee.
 

Offline JFJ

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Re: Rom options for HP 3456A
« Reply #8 on: August 30, 2018, 05:49:17 pm »
The failed device is the original MOSTEK MKxxxx ROM, I know there are some available from eBay, but by the time I've paid shipping ...

Sorry, when I said "They are still available via eBay" I was referring to the Motorola MCM68766 EPROMs. Purchasing just one, for $11.90 + $3.00 shipping, wouldn't attract any import tax.
 

Offline essele

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Rom options for HP 3456A -- Result!
« Reply #9 on: September 07, 2018, 06:13:47 pm »
A big thank you to everyone for your help ... I've built my adapter, programmed the prom, and everything is working brilliantly, it's even really nicely calibrated!

The adapter is a bit of a pain to solder, it might be easier with a surface mount PLCC socket, but hey it's not like you need to make up 100's. I had planned to make all three up, but the other proms seem to have gone back in fine!

For future reference the AT28C64B seems to be ok from a timing perspective.

I thought I had a 3456A that was beyond fixing ... so I'm very pleased with this result!

(Photos attached)

Lee.
 

Offline babysitter

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Re: Rom options for HP 3456A -- Result!
« Reply #10 on: September 07, 2018, 09:39:28 pm »
Would you open-source this?
I'm not a feature, I'm a bug! ARC DG3HDA
 

Offline essele

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Re: Rom options for HP 3456A -- Result!
« Reply #11 on: September 08, 2018, 07:34:49 am »
Hi Babysitter,

Sure, it's only a simple adapter ... I did it on EasyEDA, originally it was a DIP28 to DIP24, however I changed to PLCC (hence the incorrect URL name, which you don't seem to be able to change.)

https://easyeda.com/essele/EEPROM-28-to-24-Adapter

I do actually have quite a few spare boards, and I'm happy to post them out to anyone in the UK for a nominal cost-covering fee if anyone is interested, just drop me a PM.

Lee.
 


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