Thread: Mk14 vdu
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Old 6th Jun 2020, 12:57 am   #56
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: Mk14 vdu

This is beginning to sound quite dangerous. I'm wondering if it would be wise to have current limiting resistors (100R?) between each 8154 port pin and the corresponding VDU control input or status output.

Obviously if you were to inadvertently set 8154 PBO to output mode in direct opposition to VDU 'top page' no good would come of the resulting bit-battle, so separating them with a modest resistance would allow them to go to opposing states without damaging each other.

If you are jumping 'top page' across to one of the page select inputs to auto-switch between one 256-byte block and another, it follows that the 8154 pin connected to that page select input MUST be left in input mode so that it does not fight with the 'top page' signal.

I've put together my 'double sided stripboard' and grafted a DIN female connector onto one end, I've run the clock signal through from one side to the other and added the NRDS pullup, my edge connectors for the MK14 end are on order, hopefully they will arrive early next week and then I will be able to start to make progress on this.

In the light of your experience, maybe let's just all agree now that three out of four of b9-b12 (page selects) will be held in specific states by links, the remaining page select pin will be connected by a link to 'top page', inverse and reverse will stay linked for 'not inverse' and 'not reversed', Character/Graphics selection will be via Flag 2, and VDU enable / disable from Flag 1, possibly through a transistor inverter so that the VDU is off until otherwise informed.

The 8154 ports will arguably be much more useful for general I/O and should be disconnected from the VDU control lines and made available for connection to the outside world instead of tying them up for VDU functions.

The actual page select / top page combination will change (permanently, I think) when we all have extra memory mapped at 0200-07FF because it will make far more sense for the screen memory to be in a straight 512-byte run which is not used by the system for anything else.

Last edited by SiriusHardware; 6th Jun 2020 at 1:02 am.
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