RS232 serial null modem cable wiring and tutorial Compatibility issues The null modem cable with full handshaking does not permit the older way of flow control to take place. The main incompatibility is the cross connection of the RTS and CTS pins.Rs-232 Cables · Flow Control · The Serial UART · Hayes at Commands Amazon: null modem pinout AYA DB9 (9-Pin) Female to DB25 (25-Pin) Male Serial Null Modem Cable (25Ft) 5.0 out of 5 stars 1. $10 $ 10. 99. Get it as soon as Fri, Aug 9. FREE Shipping on orders over $25 shipped by Amazon. Only 1 left in stock - order soon.