It's not as easy as I thought...
I've used a standard USB-to-TTL like that (
https://www.amazon.com/dp/B07D9R5JFK):
View attachment 144303
and tried to connect it to the left side of the following TTL-to-RS232 converter module (
https://www.amazon.com/dp/B091TN2ZPY):
View attachment 144296
and crimped the 3-pins on the right side to the EG4 specific RJ45 pinout (1=RxD, 2=TxD, 8=GND - but no DSR signal for RJ45 pin 4!).
But I had no luck.
The special EG4-RJ45 inverter cable
View attachment 144297
uses 4-pins (1=RxD, 2=TxD, 8=GND
and 4=DSR) but all TTL-to-RS232 converter modules which I was able to find are only using 3-pins (DSR is always missing) - but all standard ready-to-use USB-to-RS232 cables (like
https://www.ebay.com/itm/255666926991, see picture below) support the DSR signal!
View attachment 144306
It looks like that the EG4 requires the DSR signal and I have no idea how to generate this signal (I've also tried to tie it to GND but this does not work). I don't want to destroy the case of my existing USB-to-RS232 cable with DP9 connector to find out how these standard USB-to-RS232 cables generate the DSR signal (on DP9 connector pin #6) because they are usually also only using the MAX3232 chip.
Does someone know how to deal with the DSR signal when converting TTL-to-RS232?