Multi-Tech Systems MT5600SMI-34 - Manual (Page 123)

Appendix B – Product Approvals, Regulatory Design Considerations, and Regulatory Compliance
Multi-Tech Systems, Inc. SocketModem MT5600SMI Developer’s Guide 123
5V Tolerant Inputs for 3.3V Modules
In order to drive the inputs of 3.3V modules from 5V logic, it is recommended to add a 1K series
resistor to each of the inputs: TXD, RTS, DTR, and RESET.