You can?I know that you can use a RS232 serial cable to control/monitor the PLC but can I use it as an input?
Can you?Can I plug a flow meter with a RS232 serial interface into the PLC and read values?
First, you need a PLC model that supports the serial I/O protocol. That means oither a VersaMax PLC or 90-30 CPU models 351, 352, or 363. Any of these have an RS232 port that can be configured to accept ASCII strings.
Information on setting up the ladder logic to support serial I/O can be found in chapter 9 of the manual GFK-0582D, available here .
If your flowmeter can be configured as a Modbus RTU master, you have more options, not to mention a lot less ladder logic debugging.
...The HMI is running on the same PC described above and was created using the Cimplicity HMI software.
RS-232 serial comms might be easier to program from that PC, although you end up in the same boat if you want to integrate data gathered via comms into the PLC?
How is HMI implemented? Do you have source code?