If you looked up “rs232 to ethernet cable pinout ” you are probably thinking of two things.
- You have a device with an RS232 port that uses an RJ45 connector (common on industrial PLCs, network gear, and access control systems). You need to connect it to your computer’s DB9 serial port.
- You want to use cheap Ethernet cable to extend an RS232 connection beyond the standard 15-meter limit, because official RS232 cables are expensive.
Either way you need to know which wire goes where.
Lets be clear: RS232 and Ethernet are different. You can’t plug an RS232 device into an Ethernet port. Don’t expect it to work just by plugging it in. However, you can use the internal wires of an Ethernet cable to carry RS232 signals. The trick is getting the RS232 to ethernet pinout right.
⚠️ Crucial Note Before You Wire: A DIY RS232-to-RJ45 cable is a passive connection. It simply uses Ethernet wires to carry serial signals up to 15 meters. It will NOT put your RS232 device onto your local IP network (LAN). > If you need to access your serial device via an IP address, or want to connect it to an Ethernet switch, a passive cable will fail (and may burn your board if plugged into PoE). For true network access, you need an active Industrial RS232 to Ethernet Serial Server .
Here’s a number that might surprise you: RS232 is still used in 30–40% of industrial equipment —PLCs, scales, CNC machines, medical devices, and test equipment. It’s been around since 1969, and it’s not going away anytime soon.
But here’s the frustrating part: 25–30% of communication failures in serial networks come from simple wiring errors. Another 15–20% come from grounding mistakes. If you’ve ever connected everything “correctly” and gotten nothing but garbage data or dead silence, you’re not alone. This guide covers not just what connects where, but why it works that way, and—most importantly—what to check when it doesn’t.
The “TX/RX Cross” Mystery: Null Modem vs. Straight-Through
The most common reason for a “silent” connection is getting the TX and RX lines backward.
Three wires are all you need for basic communication:
| Signal | Purpose |
|---|---|
| TX (Transmit) | Sends data out |
| RX (Receive) | Receives data in |
| GND (Ground) | Reference for both signals |
The Rule: TX on one end connects to RX on the other. Data needs a path, and ground gives it a reference. Why cross them? Because one device’s “transmit” must go to the other device’s “receive.” If you connect TX to TX, nothing happens—they are both trying to talk, and nobody is listening.
Identifying Your Device: DTE vs. DCE
To get the wiring right, you must identify your device type:
- DTE (Data Terminal Equipment): Like your PC or a PLC.
- DCE (Data Communication Equipment): Like a modem or some Ethernet converters.
When to cross: If you connect two DTE devices (like a PC to a PLC), you must use a Null-Modem cable to cross pins 2 and 3. It is essential to keep these roles seperate to avoid damage or communication silence.
Core Connector Pinouts: DB9 and RJ45
DB9 Connectors (The Traditional Standard)
Most RS232 devices use a DB9 connector. DB9 pins are numbered, and for standard communication, you only care about three:
| Pin | Signal |
|---|---|
| 2 | RX |
| 3 | TX |
| 5 | GND |
If both ends are DB9: You need a null-modem cable that crosses pins 2 and 3.
- Converter pin 3 → Device pin 2
- Converter pin 2 → Device pin 3
- Pin 5 → Pin 5
RJ45 Pinout (T568B Standard)
Ethernet cable uses 8 wires with standard colors. The T568B wiring is most common:
| Pin | Color | Typical Ethernet Use |
|---|---|---|
| 1 | White/Orange | TX+ |
| 2 | Orange | TX- |
| 3 | White/Green | RX+ |
| 4 | Blue | Unused |
| 5 | White/Blue | Unused |
| 6 | Green | RX- |
| 7 | White/Brown | Unused |
| 8 | Brown | Unused |
For RS232, we only need a few of these wires. You can assign colors yourself—just write down what you used.
Three Common Industrial Scenarios
Scenario 1: DB9 to RJ45 (The Console Standard)
Many devices, such as PLCs, door controllers, and network switches, use RJ45 jacks for their serial ports. To connect these to your computer, you need a cable going from the RJ45 port on the device to the DB9 port on your computer.
Standard DB9 to RJ45 Pinout:
| RJ45 Pin | DB9 Pin | Signal |
|---|---|---|
| 1 | 2 | RX |
| 2 | 3 | TX |
| 3 | 5 | GND |
| 4-8 | – | Not connected |
This wiring follows the standard DTE (Data Terminal Equipment) configuration, which is the case for almost all modern computers.
If your device is DCE (Modems or specific converters):
| RJ45 Pin | DB9 Pin | Signal |
|---|---|---|
| 1 | 3 | TX |
| 2 | 2 | RX |
| 3 | 5 | GND |
Always check the manual of your specific equipment. Manufacturers like Cisco, Digi, and Siemens often have their own pinout standards.
Scenario 2: Terminal Blocks
Common in industrial converters, these feature three screws labeled TX, RX, and GND:
| Terminal | Connect To |
|---|---|
| TX | Device’s RX |
| RX | Device’s TX |
| GND | Device’s GND |
Scenario 3: Using Ethernet Cable as RS232 Extension
The RS232 standard technically limits communication to 15 meters (roughly 50 feet). Beyond that, signal degradation and noise become significant issues. Ethernet cable (Cat5e or better) is an excellent option for extending this range because its twisted pairs help negate interference.
For Longer Runs: If you must go further using Cat5 or Cat6, you need to lower the baud rate (e.g., from 115200 to 9600) to compensate for signal loss and capacitance.
💡 Pro Tip for Distances Over 15m: Extending RS232 passively over long Cat5e runs introduces severe voltage drops and electromagnetic interference (EMI). If you don’t want to sacrifice your baud rate or if your factory environment is noisy, skip the DIY cabling. Using an active Ethernet Serial Server lets you change RS232 to TCP/IP right away. It also lets you send data over your existing LAN without losing quality, no matter how far away you are.
If both ends are DTE (computer to computer, or computer to most equipment):
| RJ45 End A | RJ45 End B | Signal |
|---|---|---|
| 1 | 2 | TX → RX |
| 2 | 1 | RX → TX |
| 3 | 3 | GND → GND |
| All others | Not connected | – |
This is a null modem crossover cable. TX on one end goes to RX on the other.
If one end is DTE and the other is DCE (rare):
| RJ45 End A | RJ45 End B | Signal |
|---|---|---|
| 1 | 1 | TX → TX |
| 2 | 2 | RX → RX |
| 3 | 3 | GND → GND |
Professional Tip: To maximize the anti-interference benefits of Cat5/6 cable, never put TX and RX on the same twisted pair. Instead, pair TX with a Ground wire and RX with another Ground wire. This gives the signal better protection and it is more stable when it has to travel a long way which is really important in noisy industrial environments.
What About Flow Control (RTS/CTS)?
The setups above use only three wires, which is sufficient for most applications as many devices don’t require hardware flow control. However, if your device requires RTS/CTS:
| Signal | RJ45 Pin | DB9 Pin |
|---|---|---|
| RTS | 4 | 7 |
| CTS | 5 | 8 |
Use the same color scheme throughout and document it. You’ll thank yourself later when troubleshooting.
The $5,000 Grounding Mistake
You have to connect ground every time. Without it, the signals have no reference. Data will be garbled or completely missing. But here’s the nuance: shield ground is not the same as signal ground.
- Signal ground (pin 5 on DB9) carries the reference voltage for the data. It must be connected.
- Connecting the shield at both ends creates ground loops—current flows through the shield, inducing noise on your data lines. This is a common but costly mistake.
A $5,000 Lesson: A factory once put in a long RS232 cable that was connected to the ground at both ends. Every time a nearby machine started, the computer screen would flicker and crash. They lost over five thousand dollars in downtime while trying to find the error. When they removed the ground from one end of the RS232 cable the problem with the RS232 cable went away away.
Systematic Troubleshooting: The Loopback Test
When your wiring seems okay but nothing is working, do not guess. Test it. The Loopback Test is a way to isolate the problem on your own without special tools.
- Disconnect the cable at the device end.
- Jumper TX to RX: Use a small wire to connect the Transmit pin to the Receive pin on the connector.
- Send Data: Use a terminal program (like Putty). If what you type appears on the screen, your converter and cable are perfect. If not, the link is broken.
Decision Flowchart: When It Doesn’t Work
Step 1: Verify Continuity Use a multimeter to confirm that TX on one end connects to RX on the other (not to itself), RX connects to TX, and GND connects to GND .
Step 2: Check Power Confirm both the converter and the end device are powered. If they are not, nothing works.
Step 3: Verify Serial Settings Baud rate, parity, data bits, and stop bits must match perfectly . A common mistake is a device at 9600 baud while the converter is at 19200.
Step 4: Check Software & Addresses If using Modbus, is the device ID correct? Ensure your software points to the correct COM port or IP address.
Step 5: Local Loopback Perform a loopback test directly on the device ports. If the device cannot read its own transmission, the problem lies within the device hardware, not the cable .
| Symptom | Likely Cause | Fix |
|---|---|---|
| No data | TX/RX swapped | Swap pins 1 and 2 |
| Garbled data | Wrong baud rate, or ground missing | Check settings, verify GND connected |
| Intermittent connection | Bad crimp or solder | Re-do the connector |
| Nothing works | Using Ethernet switch in between | You cannot put RS232 through a switch. It’s point-to-point only. |
Warning: You need to be careful when you are working with RS232 because the RS232 uses signals that can go up to plus 12 volts or down to minus 12 volts. TTL serial (used in Arduino/Raspberry Pi) uses only 0 to 5 volts. Connecting these directly will cause permanent damage.
DIY Cable vs. Serial Server: Which Should You Choose?
Making your own cable works for short distances, but RS232 has inherent limits: 15 meters maximum, point-to-point only, and no network access . If you need to monitor a device from a central control room or the cloud, a dedicated RS232 to Ethernet converter like the Valtoris 1CH-RS232/485/422-ETH is the professional solution .
A serial server lets you eliminate distance limits, access devices from any computer on the network via virtual COM ports, and ignore pinout headaches—it just works .
The Ultimate RS232 Pinout Cheat Sheet (DTE & DCE Scenarios)
| Scenario | Wiring | Notes |
| DB9 to RJ45 (DTE) | RJ45-1 → DB9-2, RJ45-2 → DB9-3, RJ45-3 → DB9-5 | Cisco console cable standard |
| DB9 to RJ45 (DCE) | RJ45-1 → DB9-3, RJ45-2 → DB9-2, RJ45-3 → DB9-5 | For modems or DCE devices |
| Ethernet extension (cross) | End A 1→ End B 2, End A 2→ End B 1, 3→3 | DTE to DTE (null modem) |
| Ethernet extension (straight) | 1→1, 2→2, 3→3 | DTE to DCE |
Pinouts are different, for each company. For example Cisco, Digi and other PLC brands have their special ways of doing things. Always consult your equipment’s manual before making a permanent cable.
If you’re tired of manually crimping custom cables and constantly troubleshooting connection errors, upgrading to an industrial serial server is your best bet. With a dedicated device, you only need one converter and a standard Ethernet cable to eliminate pinout-related failures permanently.
Frequently Ask Questions
Q: Will plugging my DIY RS232-to-RJ45 cable into a PoE switch damage my equipment?
A: Yes it is very likely to happen. Standard RS232 chips are designed to handle ±15V maximum. Power over Ethernet switches can send between 48 volts and 54 volts of direct current through the line. If you plug your device into a Power over Ethernet port by mistake, the overvoltage can instantly destroy the serial transceiver or the equipment’s motherboard. Always label your serial-over-Cat5 ports clearly.
Q: Can I use a standard “Cisco Console Cable” (light blue) for any industrial device with an RJ45 serial port?
A: No. There is no standard for RS232 over RJ45. Cisco uses a pinout called “rollover”. Using this cable on other devices—such as an APC UPS—can bridge the wrong pins, can shut down your system away or even damage the hardware. Always check with the manufacturer to get the pinout before you connect.
Q: I finished wiring my custom RJ45 to DB9 cable. How can I safely verify it works before connecting it to my industrial equipment?
A: To do this in a way you should do a local loopback test using your personal computer .Disconnect the cable from your equipment, use a small wire to temporarily short the TX and RX pins on the connector, and type into a terminal program like PuTTY. If the things you type appear on the screen again that means your cable is working correctly and pinout are correct.
Q: My cable passes the continuity test, but the data becomes garbled when the factory motors turn on. Is my pinout wrong?
A: If the pinout is correct the problem is probably interference or a ground loop. RS232 signals are not balanced. Can pick up a lot of electrical noise over long cable runs. Make sure the signal ground, which is Pin 5 on DB9 is connected properly, and if you are using shielded Cat5e cable, make sure the shield is grounded at one end only to prevent inducing noise on your data lines.
Q: I extended my RS232 connection over the network, but my legacy CNC software only lets me select “COM1” or “COM2”. How do I input the IP address?
A: Legacy software cannot natively understand IP addresses or network routes. You must bridge this gap by installing a Virtual COM (VCOM) driver on your computer. This software captures your legacy application’s local COM port requests and transparently redirects them over the Ethernet network to your converter. If you have never set this up before, you can follow our step-by-step tutorial on [How to Install and Configure Virtual COM Port Drivers]
Q: My physical wiring to the serial server is perfect, but my SCADA system still cannot read the sensor data. What am I missing?
A: Physical wiring only fixes the hardware part. If your main SCADA system uses Modbus TCP over Ethernet and your end-device uses Modbus RTU (over Serial), the data formats are completely incompatible. You have to go to your servers website and set it up to change the Modbus TCP protocol to work with your Modbus RTU end-device. For the exact mapping configuration, refer to our [Modbus RTU to Modbus TCP Gateway Setup Guide].
