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Old 5th Aug 2020, 9:08 pm   #51
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: MK14 memory upgrade

This is how the 'Bridge' PCB has evolved, it's made from two back to back bits of stripboard with a connector for the MK14 on one end and a connector for the VDU on the other.

On the underside, the side which mainly carries the bus signals, almost all signals are routed straight through one-to-one. (Image #1). The exception is the XOUT clock signal from the MK14 which starts out on the underside of the MK14 edge connector, but is jumped from there to the same track on the top side of the bridge board.

On the top side there is an untidy looking array of socket connectors allowing routing of the VDU control connections with Arduino-style plug leads.

It's not easy to see in the image (#2) but in all cases where 8154 port pins would be directly connected to VDU control pins there is a series resistor between the 8154 port pin and the VDU control pin / pluggable connector.
This is so that the 8154 ports will not be damaged if they are inadvertently placed in output mode when there is a wire link pulling the VDU control line high or low. The TOP PAGE output from the VDU also has a resistor in series with it.

Images #1 and #2 show the 'Bridge Board' as it was when it was basically just a 'joiner' which fitted between the MK14 issue VI and the VDU.

In image #3 the underside suddenly looks very busy, as a RAM IC and address decoder IC have been added to a smaller square of stripboard which has been mounted on the underside of the Bridge PCB. The bus signals still pass straight through underneath the smaller memory board, but the memory board now picks up the signals it needs from the tracks on the main lower board. Note that this image was taken before minor issues like the routing of NRDS / NWDS were sorted out, but it still looks pretty much the same.
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