Do you mean you have TTL output from some electronic circuit that you wish to connect to the PLC's NPN input? If that is the case, then the ULN2003A or ULN2803A based circuit described in the manual will work with your TTL output.
If these are simple digital I/O (not high speed encounter signal, etc) you can also use the EXP4040 which provides 16 CMOS level input. There shouldn't be any problem interfacing these CMOS level inputs to the TTL signal outputs from circuit board.
Actually unless you have a very old circuit board, the term TTL may just be a misnomer as most electronic ICs today are CMOS based. The manufacturer of the circuit board may have used the name "TTL" to describe their 5V logic level I/O.