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title: "Add serial port to your Sinclair"
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published_at: "2020-10-27T17:14:34+00:00"
modified_at: "2026-06-29T23:13:00+00:00"
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excerpt: "It is time for another hardware project: this time a serial interface, for the ZX81, 2068 (and I believe the SPECTRUM). The design is fully hardware and software compatible with the popular TS2050 (TIMEX) modem. As usual, I tried to keep the circuit as simple as possible to make it easy to construct. I have…"
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---

# Add serial port to your Sinclair

It is time for another hardware project: this time a serial interface, for the ZX81, 2068 (and I believe the SPECTRUM). The design is fully hardware and software compatible with the popular TS2050 (TIMEX) modem.

As usual, I tried to keep the circuit as simple as possible to make it easy to construct. I have build several of these units without problems.

This interface is suitable for transferring files to and from your PC (and other computers). The hardware is self explanatory and easily constructed using point to point solder connections on a perforated PCB.

To simulate a 2050, I0 address 73 selects the data and address 77 the control register. Install 0.1 uF capacitors between the power pins of each chip. I used a DB25 connector but DB9 connectors may also be used.

Although the RS232 circuit is nonstandard, it works well over short distances (3m) : add a level converter if you like to meet the RS232 standard.

To simplify connections to other computers, I have installed a jumper (J1) on the DB25 connector to tie CTS TO DSR and RTS leaving only the external connections for RX,TX and GND. The TX and RX lines of this I/F must be connected to the RX and TX lines respectively of the other unit. Someone may suggest different connector arrangements, optimized for other applications.

I use a simple ML (machine language) program to upload and download programs from the PC and I will post the program on NVG (a location on the Internet with what they claim is getting close to 3,000 Spectrum programs, as well as QL and ZX81 programs!). You may wish to use a communication program like ZXTERM80 with build-in XMODEM.

If you have any suggestions please post them: feedback is vital for a project like this.