There are budget breakout boxes for DE9 / DB9 that can be used to easily identify which pin is transmit of any RS232 device with a male or female DE9 (aka DB9) connector. For any two devices (A and B) you want to connect together, you need to connect transmit of A to receive of B. And receive of A to transmit of B.
Here's one such $18 breakout box: https://www.amazon.com/Breakout-Tester-Monitor-Female-Module/dp/B08CDQ76Q8/ref=asc_df_B08CDQ76Q8/?tag=hyprod-20&linkCode=df0&hvadid=459581405273&hvpos=&hvnetw=g&hvrand=8942761862328925844&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9005182&hvtargid=pla-979558943734&psc=1 To use that and most serial breakout boxes, turn all switches on, plug in only one side, whichever LED of TXD or RXD is illuminated is transmit from the side you have plugged in. Unplug that side then plug in the other side. And whichever of TXD or RXD is illuminated is transmit. 1. If both sides had the same LED (TXD or RXD) illuminated, they can’t talk with each other without crossing over TXD and RXD, so turn off switches and use two jumper wires to connect a) TXD on male side to RXD on female side and b) RXD on male side to TXD on female side. 2. If the two sides had different TXD and RXD LEDs illuminated, no crossover is needed. Turn off all switches except the ones for TXD and RXD. You now have transmit from one device connected to receive on the other device. Which is what you want. You may also need to connect other signal lines if one side needs RTS enabled before it will transmit data. But RTS on one device will typically correspond to CTS on the other. You don’t actually want to connect RTS of one device to RTS of the other. You can turn on switches for other signals as needed. And jumper other signals as needed. These little breakout boxes are really simple to use to find what’s transmit on each device, unless the wiring is very non-standard, which sometimes it is. -- Ron
