Yes, 74LS257, not 74LS237 The fact that the address count always stops at 192 (128+64) suggests that address bits 6 and 7 on the driver chip are defective (shorted together). The chip at U25 drives both lines and the physical location of the drivers are located adjacent to each other on the chip. This a typical failure of that kind of chip. The 74LS258 controlls refresh. Failur of this usually gives a slightly different address each time. Bill On Tue, 16 Mar 1999, Ruud Baltissen wrote: > 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 > > > - > This message was sent through the cbm-hackers mailing list. > To unsubscribe: echo unsubscribe | mail firstname.lastname@example.org. > - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail email@example.com.
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