Is serial from the cassette ports possible?

[1024MAK]

Hi Xav

I will take the same Spectrum 128 controls, but i had to data BUS pins D0=D5 to use it withe the ZX81 programs...

Sorry, I should have been clearer. Keep RxD and TxD as per the original design. Only the added CTS and RTS signals will be like the ZX Spectrum 128 (although in the Sinclair documentation they label the signals differently).

To improve the design for use with RS232 12V signals, a change in resistor values plus some extra resistors and diodes are suggested. As per this amended diagram (attached as a PDF). This provides some limited protection for the design without affecting its use when connected to 5V TTL serial systems.

5V TTL outputs will normally drive RS232 inputs okay.

Depending on what is connected, the Z80 may have to invert the relevant bits as 12V RS232 has inverted signals compared to 5V TTL RS232.

I hope this clears things up

Mark

[XavSnap]

Hi,

Many thanks Mark.

The new card is released 1.1:

Wiring diagram:

Zx81_PIO_r1.1.pdf

(63.33 KiB) Downloaded 128 times

Kicad/Gerber project here:
http://abandon.pcbs.free.fr/ZX81_International/

[gbejniet]

Huge thanks 1024MAK XavSnap ! Your time spent is very much appreciated.

This looks like a really good solution - I have edge connectors lying around so I think all I need is the logic. I'm happy to take TTL and put a MAX232 or whatever in front, that's not a problem in my mind.

(I suppose if one really wanted to drive TX from MIC one would need something like a 555 responding to pulses with a sustained level for a bit width. The above is of course a less laboured solution though.)