USB To UART With RJ‑45

This project extends a standard USB‑to‑UART converter by routing the serial data through an RJ‑45 connector. Some networking equipment uses serial interfaces but expects them over an RJ‑45 jack instead of a DE‑9 connector or header. This board combines the CP2102 USB bridge with a SN75C3221PWR line driver so that data can travel over twisted‑pair cable.

Why Use UART Over RJ‑45?

This part explains the motivation for routing serial data over RJ‑45 connectors and the scenarios where this approach is beneficial.

Many data‑center switches, routers and servers still provide a serial (RS‑232) console port. Instead of a bulky D‑sub connector, manufacturers often implement the console on an RJ‑45 to fit within dense cabling harnesses. Laptops seldom include native RS‑232, so this module provides a small USB dongle that connects to these consoles without needing an Ethernet PHY or establishing a network link.

Using The Module

The instructions here guide you through powering the module, connecting the cable and understanding the pin assignment.

• Power and enumeration – Plug the module into a USB port. The CP2102 enumerates as a virtual COM port and powers both itself and the SN75C3221 line driver.
• RJ‑45 cabling – Use a standard Ethernet cable to connect the module to the console port of your networking equipment. Only certain pins carry the single‑ended TX and RX signals, so there is no need for Bob Smith termination because the cable does not interface to an Ethernet PHY.
• Pin assignment – The CP2102’s UART pins are connected to the SN75C3221, which converts the 3.3 V UART levels to RS‑232 voltage swings. The line driver’s outputs are routed to specific pins on the RJ‑45 jack. Consult the schematic to determine the pin mapping and cable orientation.

Design Details

The design details cover ESD protection, board layout choices and component reuse that differentiate this module from simpler converters.

• ESD protection – Bidirectional TVS diodes protect the data lines on both the USB and RJ‑45 sides. Although the SN75C3221 includes some internal protection, external diodes provide additional robustness.
• Board layout – The PCB extends the CP2102 module to make room for the RJ‑45 connector and SN75C3221 circuit. The RJ‑45 jack is placed close to the TVS diodes, with enough clearance for routing underneath. All routing is coplanar with a ground pour, and stitching vias connect the top pour to the bottom ground plane.
• Connector orientation – The RJ‑45 jack is chosen with a vertically oriented latch so it can be released from the top side of the board.
• Re‑use of CP2102 circuitry – The design reuses the original CP2102 circuit from the USB‑UART bridge and adds only the SN75C3221 and connector, so routing and placement are familiar to those who built the earlier module.

By providing a console connection through a standard Ethernet cable, this module makes it easy to manage networking equipment from modern computers. Designers can adapt the pinout to match different console standards or integrate the circuitry into their own boards.

The SN75C3221 operates from a 3 V or 5 V supply and offers ±15 kV ESD protection, making it ideal for electrically noisy environments. When assigning pins on the RJ‑45 jack, follow the TIA/EIA‑561 standard for serial over RJ‑45 or the specific pinout provided by your equipment manufacturer. Keep the twisted‑pair cabling as short as practical to minimize signal attenuation. If you need to support additional control signals such as DTR or RTS, adapt the circuit to route these through unused pairs in the RJ‑45 cable.

Additional Resources

Read the full guide article on Altium Resources for a deep dive on this project. The video below gives an overview of the PCB design and how the CP2102 can be used with a line driver for RS-232 over twisted pair.