Re: is there any easy way to upgrade a C116 to 64k

On 05/01/2013 03:15 PM, Bil Herd wrote:
> I am (pleasantly) surprised that _any_ of the units really work after 30
> years given the history we had with some of the IC production (passivation,
> ion implanter) and I don’t remember where the 7500 fell in whether it was a
> finished design rule set or whether all  long-term bets are off unless
> you're on the 8500.

Well, the 8501R1 ist also not known for a long life. Back when I had my 
first C16, I removed the 7805 from the case (put it outside with a 
larger heatsink) and put heatsinks on CPU and TED. Never had any trouble.

MOS did make a 8501R4 in 1986 and there are even 8501 with a 1990 
datecode. Those are rumored to last a lot longer.


> The TED system lives with a certain amount of contention, I mentally rated
> it at the time at about 20% contention at the time  as tristate buffers of
> the day turned off much slower than they turned on so some portion of
> current was likely to occur during the bus switchover.  We used to refer to
> it as the bus heater effect meaning that we exchanged the die temperature
> (and lifetime) for time to market and unit cost.

I still have a  working 7501R1 and, surprisingly, it needs about 20mA 
_less_ power than the 8501R1 and R4.


> I remember something being said by management to make sure that the smaller
> machines weren’t easily upgradable but at the engineer level we didn't hold
> too much regard as we had all heard the stories of the alienation that
> drilling out the PET PCB's had done. I don’t think we could have gotten away
> with anything where all you had to do was swap DRAM's, though 16Kx4's were
> brand new at the time and 256k DRAMSs didn’t yet exist that I remember.

You could have put a RAM_enable signal on the expansion port without 
needing extra gates though. Both C16/116 and Plus/4 have some spare 
gates that could have been used to gate _CAS so that a module on the 
expansion port could have disabled the internal RAM. But someone in 
Management might have noticed...

As for keeping the systems operational, what I usually do besides memory 
expansion:

- Replace 7805 with switching regulator (RECOM 5V/1.5A type)
- Remove R10 (the large ceramic resistor)
- Add heatsinks to CPU, TED and PLA
- Replace the ROMs with properly programmed 27C128 or 27C256.
   (saves about 50mA per ROM replaced)

That makes the system run a lot cooler in general, also meaning cooler 
dies for TED and CPU and hopefully some more years.

  Gerrit



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