The very newest OS302 Series C FRN 11 firmware for the SLC-5/03, -5/04, and -5/05 controllers can configure the Channel 0 serial port as a Modbus RTU Master.
If your SLC was built in the past 12 years or so (i.e. not the Series A hardware), it can be upgraded with a firmware plug, part number 1747-OS302.
** I learned that some of the FRN11 firmware plugs were mis-programmed with FRN10; it might be a few days before that gets sorted out and you can buy those upgrade plugs again.
You can also use a Prosoft MVI46-MCM module, which are very popular Modbus interfaces.