ZX Spectrum - Issue with RS232 (Serial Port)

Question

I recently got hold of a ZX Spectrum +3 and am trying to get RS232 working with the spectrum. I’ve built a cable (‘BT’ style connector <-> DB9 serial) following the pin out of the cable (Spectrum 128 RS232 data cable) here. The other end of the cable is connected to the PC using a USB-Serial adapter. I'm using Moserial on Linux to communicate with the Spectrum.

The cable works and I can use LLIST to print BASIC programs over the serial port, but I'm unable to get the Spectrum to read from the serial port reliably - even when enabling hardware handshaking (DTR/CTS) in Moserial.

I wrote a simple program in +3 BASIC to print received characters to the screen. It seems the first character is received OK, but the remaining characters are dropped or become corrupted.

Here is an example of what the Spectrum outputs when attempting to send 'zx spectrum' at 300 baud:

z[dot/box character]. VAL$ [box character]MOVE VAL$ VAL$ ?)

(it seems VAL$ and MOVE$ are each a single character in the Spectrum's ROM)

screenshot:

And the BASIC program which opens the serial port, and prints received characters to the screen:

10  FORMAT LINE 300
20  FORMAT LPRINT "r"
30  FORMAT LPRINT "e"
40  OPEN #4,"p"
50  PRINT INKEY$#4;
60  GO TO 50

I discovered that if I send characters one-by-one from the PC with a long enough delay between them, I can get a much more reliable output from the Spectrum. I tested this with different delays, and 80ms worked the best. I don't really want to use this approach as a solution - it's awfully slow and occasionally some characters are dropped.

Could this be an issue with the Spectrum itself? Or am I missing something in my setup? Something just doesn't seem right, I know there is a program loader for the spectrum over serial - so surely the spectrum must be able to accept serial input without a 80ms delay per character?

Answer 1

I came across your problem as I was trying to do the same!

You need to enable RTS/CTS handshaking in your linux (PC) application, not DTR/RTS.

This works for me on my Spectrum+2:

 100 REM ** Receive Test **
 110 REM ** Use RTS/CTS **
 120 CLS
 130 FORMAT "p";9600
 140 OPEN #4,"p"
 150 PRINT INKEY$#4;
 160 GO TO 150

Answer 2

The ZX Spectrum Interface 1 works by bit-sampling, in a software loop, with delays, so that it can look for the start bit and then look for the other bits. There is no shift register. It's all done in software.They don't even use interrupts or some other kind of fixed external timer to generate baud rate delays.

As a result, it doesn't even start looking for a start bit until it knows you're looking for input... Now that you have an idea what's going on I'm pretty sure you've already seen the problem there? If you're sending data too fast, then your basic program doesn't even have time to go back to the receive routine to look for start bits, and start bits are going to be lost, or more likely, you're going to pick up a bit transition mid-byte and receive gibberish... So you need to make sure that there's a delay between bytes and your Spectrum is ready to receive the next byte before you start transmitting.

Hope this helps even though it's a while ago, but others will find this so...

Answer 3

There's software incompatibility between modern computers and ZX Spectrum when doing serial communication. It can be fixed, but not from Basic. The problem with RTS/CTS is that after Spectrum signals that it can't receive any more data via the CTS signal: modern UART chips will assume that the receiving computer is still able to receive cca. one buffer of data (=up to 32 bytes). But ZX Spectrum only has 1 byte buffer. So when Spectrum says its' full, it will get flooded by additional 32 bytes of data.

That is the reason anything faster then 1200 bauds will not work from BASIC. However ... I got ZX Spectrum to run reliably at 57600 and with error correction at 115200. By simply expecting up to 32 bytes after I signal the PC to stop sending data.

Answer 4

In zx spectrum 48k, with interface 1, i must choose how the rs232 will work. Mode t : text ->rx and tx only character in 7bit Mode b: byte -> rx and tx full byte (8 bit)

Maybe you must chek that.

On 48k with interface 1 FORMAT "b";9600 Then , the spectrum works in 8bit data at 9600 bauds.

In your code, i dont see how will works.

My best regards.

Answer 5

I've had all this working years ago with my Spectrum and the ZX Interface 1's own rs232 port, but I wrote some C program using an old Windows 386 computer.

I still have the hardware and the cables. In fact, now that I remember I had to build my own "null modem" cable and connect it from the RS232 port on the computer to the ZX Interface 1 (I have an old Sony Vaio laptop with an RS232 port).

I was even able to download TAP files to play on the real spectrum. I'll dig up what I have and get back to you.