5V Tolerant Inputs For 3.3V Modules - Multitech SocketModem MT5600SMI-32 Developer's Manual

Appendix B – Product Approvals, Regulatory Design Considerations, and Regulatory Compliance

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.
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