Multiplayer link cable

If you're talking about using a UART, then each station on the network has to go out and buy a serial cartridge, which kind of destroys the idea of cheap networking.

If you're talking about bit-banging async serial through the joystick ports, that eats up enormous amounts of CPU and is slow. A synchronous protocol runs as fast as the two computers can manage, and is also much more tolerant to delays and the like becuase you can pause reception even in the middle of a byte, whereas with async once a byte starts you need to be there for the whole thing (and any further bytes that arrive before the other end receives a flow control signal and decides to stop sending).

A transistor will act as a single gate open-collector inverter and it can be placed with ease within a sub-d shell

A diode will also do the job (simulate an open collector).

Zeeslag by MSX-CLUB WEST-FRIESLAND ( Naval Battle )
is a MSX2 game and use a link cable in port 2 with 3 wires
connect pins 6 to 6,7 to 7 and 9 to 9 of DB9

No apologies are necessary; it's perfectly reasonable to ask questions when there's stuff you don't understand. And there's a lot of lowish-level electronics knowledge involved in this you probably haven't encountered, so yeah, a bunch of it is going to be mysterious at first.

The short answer is (as you already guessed): you use register #15.

The key trick here is that there are two IO port lines hooked up to each bidirectional joystick trigger line (pins 6 and 7). One is used for input (on port #14), and one is used for output (on port #15).

You can see this by looking at the schematic, noting that when the lower '157 is set so that 1A connects to 1Y, IOA4 is reading the level on the pin 6 line, and IOB0 is simultaneously able to set the level on the pin 6 line.

Now, this is where it gets a bit hairy and the pull-ups and pull-downs come in, so while understanding the following paragraphs will be helpful, if it doesn't make sense yet maybe just leave it to come back to later, and just do your reading on port #14 and your writing on port #15.

Pin 6 has a "pull-up" on it, which is that resistor (the squiggly line leading to "+5V") attached to it. If nothing else is driving this line, it will be pulled high (to a "1" value) by this connection to the power supply.

Now the port reading this, IOA4, can't affect this: it only checks the level, never tries to change it. Whatever you read here is whatever the level of the line currently is. So per above, if nobody is actively driving this line low, it will always read high.

The port writing this, IOB0, is going through a gate (that triangle labeled "7407") with what's called an "open collector output." This is a very transistor-y name for a simple concept: when the input is high, it does nothing (not affecting the line on its output at all); when the input is low it pulls the line to ground.

So if you set IOB0 low, the line will go low; the 7407 overpowers the resistor trying to pull it high. But if you IOB0 high, the gate will have no effect on the line on its output and it will be high or low depending on what else is trying to affect it.

If nothing else is trying to affect it, the line will be high, because of the pull-up resistor. But maybe pin 8 is connected to pin 8 on another machine, and that one has set IOB0 low, so that would pull the line low (which you would read on IOA4). And of course the same works in reverse: if you have set your IOB0 low, that's going to pull the line low and everything connected to it, including the IOA4 on the other end, will sense that.

Phew, this was all a bit, and I'm not sure how much sense it made.

TLDR:

Assuming you're using a shared trigger line: set IOB0 high when you want to see what others are writing to the line, reading that on IOA4. Set IOB0 low when you want to signal others on the line.

For unidirectional lines, hook up either trigger or pin 8 to any of the input pins 1-4 on the other computer, and then do the usual read through port IOA and write through port IOB.

You can fiddle with the following circuit and experience the reaction of the logic levels at register 14 while you click on the H /L labels on the register 15 to change logic levels.

The circuit is not the same as the MSX port but the behavior is equal.

You can take a look at this implementation of an I2C driver on the joystick port that was used to read the Wii Nunchuck controller.

https://github.com/Danjovic/MSX/blob/master/J2C/NunJ2C.asm