john/lori wrote: > > > the x501 might > > need an additional gate on the R/W line that is controlled by the "gate in" > > pin. Though my machines were working fine even with the gate in disconnected, > > which affected the R/W timing only a little bit. > > Do you know or is it known exactly what gate in does? The best information I ever read about this input is from the Plus/4 service manual: It gives the explanation "R/W gate", that's all. But that's really what it does, I checked it with a DSO (digital sampling oscilloscope). I think I still have the measurements somewhere, but I don't remember the details. I think it is needed to synchronize the R/W signal with the weird double clock timing in the 264 machines, but I don't know for sure of course. > > PS: I have headaches when I think about the recently discovered behaviour of > > writes/reads to unconnected bits of adresses 0/1. IIRC, the data direction > > bit is connected, but the data port bit itself is not. > > why would this be a problem? If you write to an unconnected bit, the written value is returned on a read for a certain time. It's about the same behaviour you get with the write only registers in the SID, and those *have* been used in software. It might be used as a seed for a random number generator, and who knows what happens if it doesn't behave like a real 8501. I'm sure the number of affected programs is near zero, but still... Even emulators are down to this level already, so why cripple a real machine ;-) Nicolas - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail firstname.lastname@example.org.
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