EDIT: After some clarification...
The intended setup is only possible with two serial ports on the PC/laptop.
The MAX232 has two amplifiers to convert from the CMOS/TTL voltage level (wire to observe) to the RS232 voltage levels of the serial cable. But, the serial port of the PC/laptop (aka. DTE) has only one receive data pin (
RXD, pin 2 at DE-9 connector of the DTE). The other data pin (
TXD) is driven in the opposite, and thus wrong, direction.
The wire to observe must be connected to the
T1IN (pin 11) or
T2IN (pin 10) input of the MAX232. The level-shifted outputs are then
T1OUT (pin 14) and
T2OUT (pin 7), respectively. One of these outputs must then be connected to pin 9 of the DE-9 connector of the DTE.
This is also a sort of a null modem link. Thus, one must connect
DTR (pin 4) to
DSR (pin 6) and
DCD (pin 1). As well as
RTS (pin 7) to
CTS (pin 8). All pin numbers are given of the DE-9 connector of the DTE.