The MEGA has a male 25pin serial connector. To connect it to a PC, you will need a cable with a 25pin female and a (most probably) 9pin female connector. TxD and RxD need to be crossed.
The cable of unknown configuration that had come with the machine somehow got lost in one of my relocation (to the fine country) boxes, so I had to make a new one. I made it from a shielded standard serial extension cable by exchanging the male 25pin connector to a female and crossing TxD and RxD (pins 2 and 3), when the control program did not recognize the MEGA. This information is not in the manual I have, it just mentions a 9 to 25 pin both sides female RS32C-V.24 cable.
My cable has these connections:
Signal | 25pin - 9pin | Signal ---------+---------------+-------------------------- RxD | 2 ----- 2 | TxD Transmitted Data TxD | 3 ----- 3 | RxD Received Data RTS | 4 ----- 7 | RTS Request to send CTS | 5 ----- 8 | CTS Clear to send DSR | 6 ----- 6 | DSR Data set ready GND | 7 ----- 5 | GND Ground CDT | 8 ----- 1 | CDT Carrier detect DTR | 20 ----- 4 | DTR Data terminal ready RIN | 22 ----- 9 | RIN Ring
For details about RS232 (serial) communication, refer e.g. to Wikipedia.
The communication parameters most probably are 9600 baud, 8N1, no flow control: After running a random move diagnostic test, I quit the CP and opened COM1 with RealTerm. Set to 57600 baud 8N1, it reported some characters and communication errors, so I decreased the baudrate (57600 baud is unusually high for a machine developed in the 90s) and no errors were displayed at 9600 baud.
Luan78zao's Blog 
Thanks for the write-up. I’m trying to communicate with an old Tecan Megaflex ID, so these comm settings were helpful. Now to see if my USB to Serial cable works…