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Old 15th Apr 2019, 12:42 am   #164
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default Re: MK14 schematic revisions

... Just caught up with your latest MK14 article finds.

That one with the image of the super-early-version MK14 is great, I don't think I have ever seen that one before. I have never actually heard of, or seen, an actual issue I MK14. All the ones I've ever seen are issue II or later.

Yes, an acknowledged weakness of the cassette audio signalling format is that it starts sending significant information immediately - this was addressed in later Sinclair machines by sending a long preamble of steady tone to let the recorder AGC settle before starting to send significant data.

I've never heard of the interesting HF injection method to pre-adjust the AGC action before but another forum member here (Gert?) had the brilliant idea of inverting the data before sending it to the cassette interface so that the signalling is done, not by silence broken by 4ms or 16ms bursts of tone, but by interrupting the otherwise continuous audio tone for either 4ms or 16ms. That would provide a lead-in tone for the recorder to adjust its gain on before any significant information begins to be sent.

I was actually wondering if we could (or should) collect all the links to known sources for MK14 software and hardware projects into a thread of its own, which might also incorporate some discussion about actually programming the machine - there's a lot of interest in recreating the hardware but people rarely write anything new to run on them (I am as guilty as anyone else).

I've been contemplating the idea of setting up a small model railway circuit and trying to do some basic PWM speed control and automatically starting from / stopping at a station - I have some (N gauge) sectional track and a few models to put on it, but no electrically controlled points, so it would have to be quite basic. I think this is the sort of experimental Arduino-like hobby control application the MK14 was originally aimed at.

Another, less physically complex and more self contained project might be to connect an alphanumeric LCD display to one or both of the 8154's I/O ports and recreate the 'Message' program with the message scrolling on the LCD display with no crippling literary limitations imposed by there being no Letters 'K', 'M','P','Q', 'V', 'W', 'X', 'Y', and 'Z'.
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