I work in the valve automation and have mainly used Hart communications over the years. I have a few customers now that are using Modbus. What do I need to be able to communicate with stand alone Modbus installed devices?
I presume that you're really communicating with either an electric actuator or a positioner because the metal or plastic valve bodies don't talk digital protocols and only sound off when they cavitate.
For testing purposes, like, writing 'drive open', 'drive closed', or 'stop' commands, or reading 'position feedback; you can use a generic Windows Modbus Master like Modscan, Modpoll, or SimplyModbus (there are others, too) with a USB/RS-232 or USB/RS-485 modem, or use Ethernet if the unit has Modbus/TCP.
I saw one shop where they supported a number of actuators on a Modbus RTU network. They used an HMI panel with Modbus RTU as the master and programmed buttons for control testing and displays for feedback.