This is what I had on show yesterday.

Cheap ESP8266 module, used in today's interactive toys for kids.
Converterted to be Rverter compatible.
So you can use the module the same as an Rverter.

One problem, when you buy it, it is set to 115,2 kBaud....
(Pokey-divider-1)
Most Atari's can't communicate at this rate due to the hardware layout of the SIO-port.
The first command you need to send is to get the baudrate down to 2400
(AT+UART_DEF=2400,8,1,0,0 )
Do not, I repeat DO NOT set the baudrate to 1200, there is a bug in the module setting it unusable!
If you want 1200 baudrate, the best one you can set is 1225, that will do fine for 1200.
If you want to TRY other baudrates, use the CUR command, this will not save to flash.
When you scewed it up, just power cycle and the default is reloaded.
(AT+UART_CUR=9600,8,1,0,0 "Example to set 9600 until next power-cycle")

The module accepts AT-commands like an old modem.
Not easy to set it up, but you need to get used to it.
There is alternative firmware to flash into the module that makes a "phone-book", but did not try that yet...

From a terminal window send this, after each line CR and LF that is control-M and control-J on atari.
So AT control-M control-J will send "AT" and it will reply with "OK".
This is the test-command.

SETUP:
AT
AT+GMR
AT+CWMODE=1
AT+CIFSR
AT+CWJAP_DEF="replace with wifi name","replace with password"
AT+CWAUTOCONN=1
AT+UART_DEF?
AT+UART_DEF=2400,8,1,0,0 set comm params
AT+CIPMODE=1 transparent mode or everything get ipd prefix
AT+CIPMUX=0 no multiplexing
AT+CIPSTART="TCP","broadway1.lorexddns.net",23 this is one example
"irata.onlne",8005 for plato
AT+CIPSEND to start sending your stuff to the server

WiFi: 2.25 euro
https://www.aliexpress.com/item/TZT-ESP ... 4c4dTZobDH

Plato:
https://github.com/tschak909/platoterm6 ... 1005-ATARI

Schematics:
See attachment.

ESP8266 communication commands:
See attachment.

Information about aliexpress:

When you buy on aliexpress, the left "COLOR" is the complete set.
So Breakout-adapter + ESP-module.
USD 2.55 about EURO 2.25
Free shipping

Das sollte sich von der Größe her ja ideal für einen Atari mit "WiFi inside" eignen.
Spricht eigentlich was dagegen, so etwas parallel an den SIO-Bus zu hängen?

sorry, just noticed the OP was in English:
That should allow for Ataris with "WiFi inside"! Is there any disadvantage in soldering this onto the SIO bus in parallel for an internal solution?

No. there is no disadvantage except you can not unplug the device.
So the motor-line is in conflict with other devices that use it.
But you can insert a double switch that powers the module and connect motor-line.
That way you can switch off the device and use a cassette-recorder.

The main concern is the baudrate of the device out of the box.
To do 115k2 you need to remove the 1pF caps on IN and OUT.
C77/C78 in an 800XL. I found this on Lotharek's homepage.
I tried 115k2 on an original 800XL and the only thing you get is garbage on the AT-command.

The other option is to set the correct baudrate on the PC using a terminal-program and a USB-TTL-UART-device.
These cost about 1-2 euro with free shipping, that is what I did.

I will do some test to see if I can enhance the IO-signals.
So that it will be possible to set/change the "out of the box baudrate" from the Atari.
The terminal software I wrote can handle 115k communication.
Now it's time for the pokey to understand, without modding the hardware internally...

More news later this week.

I build this yesterday to test 115k communication.
Instead of the 4050 (buffer) I used a 74ls04 (inverted/buffer).
Knowing that 74xx chips give better shaped serial-data then 40xx-cmos.

Result:
The ESP understand what the atari says, but the reply is garbaged by pokey.
But that is no problem, once the baudrate is set back to a normal level, the device is ready.

So, the next step is to write a small tool/program that does the "out of the box" changes for you.

Current pre-view of the ESP8266 driver/handler

Out of the box works nicely, setting the baudrate from 115k down to 1200.
To be more precise, 1221 baud exactly, due to the ESP-firmware-bug.
This "AT+UART=1221,8,1,0,0"-command is automatically generated and send after pressing the B-key.
The ERRO message is from Altirra (2nd picture).
Normally you get here garbage from the pokey (first picture).
After the garage, the ESP is set to 1200 baud and works perfectly !


But you can set any baudrate you like using the correct AT-command.


You even can get online with the build-in Tiny Terminal

I ran into some issues with PAL and NTSC consoles.
It works on one system and not the other and visa-versa....

So I dropped the "Out of the box"-option from the Tiny Terminal Mode and wrote a separate tool.
This tool will send out AT+UART=1221,8,1,0,0 with 115k2 on PAL/NTSC-systems.
As you perhaps know, the Pokey does 111k and 127k, both are out of specification to be understood as 115k2.
I used a little trick to synchronize the serial-data.....
Tested it all using my Saleae Logic Analyzer, how I love this device !

Program is tested, works, so here it is.
This will get your ESP out of the box and set it to 1200 baud, ready for action.