Re: 192 bytes free

From: Ruud Baltissen (
Date: 1999-03-16 21:04:00

Hallo William,

> Check the PLA at U17, the 74LS237 address multiplexers at U13 and U25,
> and the 74LS258 multiplexer at U14.  The 74LS237 at U25 is the most
> likely.

I presume you mean the 74LS257s. Anyway they will be the first ones to be 
exchanged (some day) and if it proofs one of them is the troubleshooter then I 
still don't have a good explanation for this behaviour. A 257 is nothing more 
than a couple of switches. So I expect a "nice" address like 1, 3, 7, 15 etc 
bytes free. 192 doesn't fit in this scheme. 
The only ICs I did not exchange yet are all the 74s and the colour-RAM. 
(Geoffrey: if no reasonable explanation then the PLA is in 99% the first one to 
be exchanged in my experience) The last one cannot be the troubleshooter and 
the same way of logica also applies to these 74s: this "weird" address doesn't 
fit in the these 74s normally behave.

Groetjes, your puzzled Ruud

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