Post the exact part number of the Panelview you have and the correct cabling can be selected. The DH485 protocol can (and does) exist on RS232 h/w, the part number will then dictate what cable and/or converter you'll need. The 5/03 doesn't support DH+ so that will introduce another set of issues.
Actually you can have DH485 over RS232 or RS485. It is a very often confused subject
Give us the part number of your Panelview. That is the easiest way to make sure what you need.
On an SLC channel 0 is physically RS232 and channel 1 is physically RS485. Channel 0 is capable of DF1 or DH485. Channel 1 is capable of DH485 only
The following are all DH485 protocol Panelviews. To the right of the part number is the physical port type.
2711-T5A2 RS485
2711-T5A3 RS485
2711-T5A5 RS232
2711-T5A9 RS232
Edit - Well jstolaruk beat me to it but I threw a few part numbers in there too
Compatibility with other equipment that runs DH485 over RS232. That's the biggest reason I know. Not real sure without knowing the particular application. There is quite a bit of products that run DH485 over RS232.
Just remember there a two parts to the serial. You have the physical and the protocol. Physical would be RS232 and RS485. Protocol would be DF1 and DH485. The fact that they used the 485 number for both the physical and the protocol was a mistake and causes a lot of confusion
I wouldn't do it on a new application but then again I wouldn't do a serial protocol on a new product.