Understanding ZX81 signals to Timex/Sinclair Printers

[Zebra Systems]

Hi. My name is Stewart Newfeld. I live in Brooklyn, New York. My company Zebra Systems, Inc. supported Timex/Sinclair computers in the 1980s, but I have not played with them in decades.

I want to start an open source project to understand and document the signals that the ZX81 provides to the AlphaCom 32 and Timex/Sinclair TS2040 printers. That would be Phase I. Putting a logic analyzer on the bus between a ZX81 and a printer would be one approach. Studying the ZX81 ROM code for the printing functions might be another. Or maybe, someone on this forum knows something about this subject.

Phase II would be to generate those signal signals from Arduinos, Raspberry PIs, microbits, etc., to drive the printers from those microcontrollers.

Phase III would be to design an ATMEL or ESP32 based serial to AlphaCom 32 and TS2040 adapter to make it easy to interface almost any comptuer / microcontroller to those old printers.

Everything learned would be open source, software and hardware. If the project ends of with an adapter, the KiCad PCB drawing and schematics, the software ... all completely open source. I am prepared to spend some money to fund prototypes.

If you know anyone who might be interested in this project, please pass on the message.

[Paul]

I am interested in this development from a different point of view.
I'd like to have a ESP32 doing the opposite. Acting like a Alphacom printer on a ZX81 or Spectrum and offering the printout via it's WiFi server for easy Access.
It will need the same understanding of what goes on on the Zeddy Expansion port including timing.
Kind regards
Paul

[mrtinb]

A printer emulator as you describe Paul can be achieved with a Memotech Centronics interface and RWAP’s RetroPrinter. https://www.retroprinter.com/

But I assume a ESP32 directly on the ZX81 would make a cheaper interface than Memotech I/F + RetroPrinter + Raspberry Pi.

[1024MAK]

The ZX81 (and ZX Spectrum) ROM code is designed to drive the Sinclair ZX Printer. So to fully understand the system used, you really need to see how the ZX Printer works. The ZX Printer burns the conductive metal coating off the ‘paper’ as a stylus moves across the ‘paper’. The stylus is moved by a motor. The same motor also drives the paper forward. The data is not parallel, but ‘bit bashed’ serial to draw an image on the paper as the stylus moves across the ‘paper’.

The AlphaCom 32 and Timex/Sinclair TS2040 printers (which I understand are fundamentally the same device) use the signals intended for the ZX Printer and instead convert the signals into a type that can drive an array of thermal heads. These heat up the thermal paper and this is printed on the paper one dot line at a time.

The ZX Printer also feeds back to the computer on where the stylus is (in a crude way), so that the ROM code synchronises the printing to the same (more or less) horizontal place each dot line.

Mark

[RWAP]

A printer emulator as you describe Paul can be achieved with a Memotech Centronics interface and RWAP’s RetroPrinter. https://www.retroprinter.com/

But I assume a ESP32 directly on the ZX81 would make a cheaper interface than Memotech I/F + RetroPrinter + Raspberry Pi.

Yes, my idea would be to have a very simple interface which sits on the ZX81 bus and passes the printer data to the Retro-Printer.

The problem is that indeed, we need to understand what signals are used to drive the ZX Printer so it can be emulated within the Retro-Printer

[1024MAK]

ZX Printer

This post details the ports and bit data for the ZX printer. The ZX81 and ZX Spectrum ROMs support three BASIC instructions (LPRINT, LLIST, COPY) which access the external printer hardware by these I/O addresses.

Port FBh Read - Printer Status

Port FBh Write - Printer Output

Horizontal resolution is 256 pixels (32 characters).

Mark

[Zebra Systems]

Thanks to everyone above for such good responses, especially to this recent one from 1024MAK with the Port FBh printer function listing.

[XavSnap]

[Zebra Systems]

Thanks XavSnap for the schematic. I think it will be helpful in figuring this out. I see they step up the voltage to 50V to zap the silver paper. I wondered how high a voltage would be needed for that.

Although the AlphaCom 32 /TS2040 printer must be conforming to the same bus interface and should not really provide additional information about the bus, I'd love to see the AlphaCom schematic if anyone can provide that.

[XavSnap]

Hi,
Something's wrong in the "official" wiring diagram:
I have not the right Ferranti name to get the datasheet…
1) The "D0" pin name is missing.
2) The "R5" is named twice.

[Edit] Redrawed wiring diagram updated in next messages...