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SiriusHardware · 7th Mar 2020, 8:34 pm

Yes, that was the last major change as I recall it, the squeezing in of an extra IC so that you could remove the PROM images while still retaining the alt-RAM feature.

The removal of the PROM images from 0200-07FF combined with the routing of the buses to the rear edge connector creates the possibility of making a RAM pack, in finest Sinclair tradition, which just plugs into the rear edge connector for an instant extra 1.5K of memory.

At its simplest, this would be a single 8-bit wide static RAM and an address decoder, but getting progressively more complex, it could have settable links so that parts of the RAM would be paged in when the 512 bytes of onboard RAM and 'Extra RAM' region addresses were activated, removing the need to find obsolete, expensive 2111s, etc.

Taking that one step further, the PCB could also have a socket on it for a 2716 EPROM which would drop in whenever an address in the remaining PROM image ranges was accessed, thereby also removing the need for difficult to obtain / difficult to program bipolar PROMs.

Finally, any such add-on would ideally need some bus buffering and of course a one to one DIN connection on its own rear edge for the MK14 VDU.

Another good touch that I noticed was the replacement of one of the two 0V connections on the keypad edge connector with a reset-input line so that an external reset switch can easily connected, especially useful if the PCB will be mounted in an enclosure with a remote-mounted keypad and display.

7th Mar 2020, 8:34 pm	#280
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,575	Re: MK14 schematic revisions Yes, that was the last major change as I recall it, the squeezing in of an extra IC so that you could remove the PROM images while still retaining the alt-RAM feature. The removal of the PROM images from 0200-07FF combined with the routing of the buses to the rear edge connector creates the possibility of making a RAM pack, in finest Sinclair tradition, which just plugs into the rear edge connector for an instant extra 1.5K of memory. At its simplest, this would be a single 8-bit wide static RAM and an address decoder, but getting progressively more complex, it could have settable links so that parts of the RAM would be paged in when the 512 bytes of onboard RAM and 'Extra RAM' region addresses were activated, removing the need to find obsolete, expensive 2111s, etc. Taking that one step further, the PCB could also have a socket on it for a 2716 EPROM which would drop in whenever an address in the remaining PROM image ranges was accessed, thereby also removing the need for difficult to obtain / difficult to program bipolar PROMs. Finally, any such add-on would ideally need some bus buffering and of course a one to one DIN connection on its own rear edge for the MK14 VDU. Another good touch that I noticed was the replacement of one of the two 0V connections on the keypad edge connector with a reset-input line so that an external reset switch can easily connected, especially useful if the PCB will be mounted in an enclosure with a remote-mounted keypad and display.