Your DMX controller's front end speaks ASCII, not Modbus, so since it's not going to understand the Modbus commands sent to it, why bother?
PLCs are great at what they do, and terrible at what they don't, speaking ASCII is squarely in the "not so great at" column. If the only reason that you are installing a PLC is to control the DMX device, then scrap the PLC and Modbus plan and go with a stand alone SCADA controller (I recommend AdvancedHMI, Irma free, and it works great).
If you have any distance between your SCADA controller and your DMX device, then use either RS485 and a RS485 to RS232 gateway, or run Ethernet over to it and use a Ethernet to RS232 gateway instead.
If you do need a PLC, then use it for what you need it to do, and keep it away from the DMX device. You can use AdvancedHMI to control it as well.
I do this for a living, I work for one of the "Big three" automakers, and have worked for Schneider Electric where I worked on Modbus fairly often. At another company, I wrote my own Modbus driver for both the SCADA side and the PLC Modbus code. So it's conceivable that I might know something of which I speak.
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