Funny chat on two ZX81 16k.

[XavSnap]

Hi,
If you have two ZX81, you can experiment a funny setup.

Take two computer, link the EAR to the MIC and the MIC to the EAR.
Launch this "P" file on both computers.
Set the first as "Master", the second one as a "Slave".
Take an very cleaver friend on the second ZX81...
And ... it's done... Your ZX81 is a "AI CHAT-ZX R1" computer !



This code is issus from the "Graphics toolbox"
But can be diet to fit in a ZX81 1k...

ZXCHAT.P

(2.03 KiB) Downloaded 44 times

Have Fun !

[XavSnap]

I will be curious to know if a Skype session can "talking" to another ZX81... if you have only one computer.

[Moggy]

I've tried linking ZX81s this way in the past, probably one of the first things I ever did outside of the norm, and it doesn't work as the signal the MIC port puts out is not strong enough without amplification to be read by the second ZX81s EAR port.

[1024MAK]

I was wandering about how the two Zeddies were being linked.

The attenuation of the signal in the circuit for the mic socket is substantial, hence the resulting signal is a very low voltage (weak).

Meanwhile the ear input needs a relatively high voltage for the ULA to detect it, meaning it needs a strong signal.

As Moggy says, an amplifier is therefore needed.

Mark

[XavSnap]

Ok, in this case, two mono amplifier are need.
One per EAR sound output.

Yes, in this case, the amplifier is a tape recorder (REC+PAUSE keys): (pdf-FR)

Reliez_2_Zx81_MicroSystemes29.pdf

(8.37 MiB) Downloaded 51 times

EDIT:
Or place two mono amplifiers: (2x1$=2$)

Capture d’écran_13-6-2024_63912_www.aliexpress.us.jpeg (18.65 KiB) Viewed 897 times

https://www.aliexpress.us/item/3256806185357515.html

3.5mm mono cable to cut and connect to the terminals (1x1$)
https://www.aliexpress.us/item/3256804866242048.html

A 3 to 12v DC power supply

[XavSnap]

The wiring diagram:

ZxLink.pdf

(19.28 KiB) Downloaded 42 times

[Moggy]

I don't think this will actually work, Xavier.

The amplifier unit you have pictured is a simple LM386 POWER op-amp and whilst it may just have the grunt to power a signal into the Zeddy it requires pre-amplification at the input usually at a domestic line level of about 2-300 millivolts and rest assured the output from the Zeddie's MIC socket is nowhere near that and would, I suspect, require more than one pre-amp stage.

Just as an example I have built a small device to make the Spectrum's output a bit louder than the beeper and the device I used is a type of power amp that requires no pre-amplification and has a sensitivity of about 40 millivolts. This works wonderfully with the Spectrum but when used with the Zeddie's MIC socket the signal is so weak it doesn't even register.


In short the LM386 is just not sensitive enough at its input and electronically speaking is a bit of a noisy beast even fitted with what I suspect are components for a simple Zobel network which may or may not help to tame any high frequencies/spikes, so would require either battery power or a very well smoothed and regulated power pack.

I don't know if the power pack in your link is suitable as all I get is a blank page when I try to open it but anything priced at $1 is unlikely to be any use.

[XavSnap]

Hi Moggy,

Yes, you're right.
You need to try to remove the pin 1-8 capacitor to drop down the gain from x200 to x20, and boost the output power (or use a 12v DC power supply).

wiring diagram:

This component :

[Moggy]

I don't think you understand my meaning Xavier.

Leave the output at X200 that's ok, it's the INPUT to each module that's the problem not the output.

By removing the capacitor you are lowering the gain of the chip down to the X20 level its own internal bias circuitry provides why would you want to do that?

In simple terms the input to the LM386 will not hear the ZX81 as the MIC output is just too weak for the LM386's INPUT to "hear" it without pre-amplification.

There is nothing wrong with the OUTPUT of either module.

Using a larger power pack solves nothing, it just makes the ceiling before the chip starts to clip higher. whether you use 5 or 12 volts if the input signal is non existent then there is nothing to amplify.

[XavSnap]

Hi Moggy,

The pre-amplification can be... no way.

The magazine tel us to get the "EAR" signal on the TAPE/TV ULA output.
May be the better way to get it, but we had to open the black beast.
Less funny in fact.