for the (older/classic) PLC-5/15 you will REQUIRE a DH+ connection ... specifically you can NOT use just a serial-type cable ... note: that handy looking little 9-pin connector on the front will NOT (I repeat NOT) let you use inexpensive RS-232 communications ...
in some cases, it's cheaper to just upgrade the old-style processor than it is to invest in the communication hardware ...
the "text book" answer to your question is:
you need a 1784-PCMK "credit card" for the laptop's PCMCIA port ...
AND ...
a 1784-PCM5 cable to match the PCMK card (be careful with the series A and series B versions - they will NOT interchange) ...
AND ...
a 1784-CP7 adapter ...
do NOT let the supplier talk you into a "modern" 1784-PCM6 cable ... the "good news" is that this one doesn't need the adapter ... the "bad news" is that you won't be able to remove the adapter to plug into the 9-pin port on the front of your classic PLC-5/15 ... specifically, the classic processors don't have the little round 8-pin miniDIN connector featured on the PCM6 cable ...
in the past you could get all of the above listed parts as a kit ... ask for part number 1784-PCMC if it's still available ... the last time I checked (years ago) the price was about US$1,500.00 ...
so ... once you've got the older PLC-5/15 covered (as listed above), you can use the same cable to connect to the new models too ...
or – if you ONLY need to communicate with the new model, then all you really need is a simple RS-232 cable ... 9-pin-female on one end to a 25-pin male on the other end ... the official part number is 1784-CP10 but you can build your own if you like ... we'll look up the pinout if you're interested ...
possible exception: sometimes the processor's RS-232 port has been reconfigured for use as a "barcode reader" port or something along those lines ... if so, you'll have a hard time trying to program through it ... let us know if that's your problem ...