How to add a 3.5 character (MODBUS-RTU) delay after the stop bits. Is that a guard time?

[TReed.1]

I'm using an STM32L486 as my master and as the slave. My issue is the slave transceiver I want to use pulls the RO (RX on the STM32) pin down after receipt of a message. The RTOI doesn't fire until the RX pin goes high from the next message.