MSX WiFi via RS232?
I have one of those but I haven't used it yet as I need to get a terminal program up and running. The customer support at Simulant so far has been awesome. Hoping to do some BBS surfing once It's up and running 
This is a picture of the MT-Telcom II modem with wifi-add-on:
... a fairly simple add-on. For now works only with a terminal program (for MT-Telcom this is inside the built-in ROM); Wifi can be accessed using standard NodeMCU commands. An example:
@dmr how did you get the Lua console on MSX? 
is it built into the wifi addon?
... finally a found some time to experiment with this approach. I have now a Proof of Concept, doing IP on an MSX via a modified serial modem (an NMS1250 this time), using Erik Maas' Fossil Driver in MSX-DOS:
All the heavy wifi stuff is done in the ESP with a simple LUA script:
uart.setup(0,2400,8,0,1,0,1)
uart.on("data", "\r",
function(data)
if data=="hello\r"
then uart.write(0, "Hello, world")
elseif data=="getip\r"
then
ip, nm = wifi.sta.getip()
uart.write(0, ip)
elseif data=="getnetmask\r"
then
ip, nm = wifi.sta.getip()
uart.write(0, nm)
else
uart.write(0, "Huh!?")
end
uart.write(0,"\n\r")
end, 0)
And on the MSX a simple Pascal testprogram (a modified TERM.PAS that is delivered together with the Fossil.INC):
BEGIN
IF TestDriver=TRUE THEN
BEGIN
Set_Speed (4,4); {2400 bps}
InitRS232;
SendString('getip');
SendByte(13);
REPEAT
data:=RecByte;
Write(char(data))
UNTIL (data = 13);
DeInitRS232;
END
ELSE WriteLn ('*** Wifi adapter not found!',char(7));
END.
... still very beta, but it works. This setup makes it possible to do filetransfers/webpage (HTTP), DNS lookups, etc. (No PING though :-O ).
Does this WiFi module have hardware flow control? Otherwise the MSX probably will loose data under heavy load.
ESP-01 doesn't have flow control. The best way I've found out to make it work properly with a MSX was to have a custom firmware made for it using Arduino IDE with ESP8266 support. In case of SM-X I've implemented full UNAPI support in the ESP8266 firmware and the UNAPI driver is a simple tunnel sending the UNAPI commands and parameters and receiving back what it need to respond. The TCP-IP stack on Arduino IDE ESP8266 library is pretty good and handles the data not read scenario correctly, closing the tcp window, and also have bear ssl support, so it is possible to have a single ssl connection 
[..] The best way I've found out to make it work properly with a MSX was to have a custom firmware made for it using Arduino IDE with ESP8266 support.
Yes, Arduino firmware can do the trick. However, I use NodeMCU firmware. This allows me to update new versions of the lua-code to the esp via wifi and NodeMCU allows easy event-driven programming. I have now a simple HTTP-server, a telnet server and a UDP "server" running simultaneously on the ESP (each in different Lua-scripts, optionally triggered during startup) as well as a serial-server to communicate with the MSX.
The HTTP server is used to configure wifi remotely via "http name_of_esp/config.html", and store the config on the flash of the ESP. The telnet server is used to transfer Lua-code to the ESP (and this can also be used to upload new wifi.cfg, which is a simple textfile).
Apart from the getip.com and getmask.com I have yet to add more commands to make it more useful (but that is just a matter of time 
)