Quote:
Originally Posted by SiriusHardware
As discussed originally, just fit a resistor permanently in series with the TOP PAGE output so that if it ever does clash with an opposing output no harm will come of it. Maybe around 330R, low enough so it doesn't weaken the drive from TOP PAGE too much, but high enough to withstand 5V across it if it ends up with +5V from an output on one end and 0V from another output on the other.
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Indeed to protect the 8154 but, I want to swap between different display RAM configs so think maybe I need a couple of AND gates instead (so an LS00 with the second pair used as a NOT?) so that when I set the bit on the INS8154 it means start driving that line onto that PS input which is combined with the TOPPAGE signal in the gate input, so only if it is asserted does that PS have an effect.
The only really useful settings are then - here the 1 means an AND would connect that line to the TOPPAGE
Code:
ST 0x05(P2) ;(b12) PS4=0
; ST 0x14(P2) ;(b9) PS1=1 (Toppage enable for Exp RAM)
; ST 0x03(P2) ;ReversePages=0 - So 200 is above 300
; ST 0x12(P2) ;(b10) PS2=1
; ST 0x01(P2) ;(b11) PS3=0 (NO toppage connected)
and for Base RAM
Code:
; ST 0x15(P2) ;(b12) PS4=1
; ST 0x04(P2) ;(b9) PS1=0 (NO toppage connected)
; ST 0x13(P2) ;ReversePages=1 - So F00 is above B00
; ST 0x12(P2) ;(b10) PS2=1
; ST 0x11(P2) ;(b11) PS3=1 (Toppage for Base RAM)