Connect the panel's RS-485 port to the drive's RJ45 socket on pin 2 (RXTX+), pin 3 (common) and pin 7 (RXTX-). Set both ends to 19200 baud, no parity, 8 data bits and 2 stop bits, and build register addresses as 4MMII, so menu 3, item 8 is register 40308. Over Ethernet, a passing ping with a failing protocol test means the wrong driver: pick the Modicon Modbus TCP/IP Ethernet driver.
An HMI touch panel talks to a variable frequency drive over Modbus RTU once three things line up. The RS-485 conductors land on the right pins of the drive's serial socket, both ports run matching settings, and the panel's tags use the drive's register convention. This article wires the serial path first, because the drive's standard port offers it without extra hardware, then covers the Ethernet variant. Over Ethernet, a passing ping hides a wrong driver selection. The pins, the settings, the register arithmetic and the fault checks are spelled out below, so you can wire the link and prove it in one sitting.
What do you need before you wire a panel to a VFD?
You need a three-conductor cable with an RJ45 plug on the drive end, a panel with an RS-485 port or an RS-232-to-RS-485 converter, the drive manual for its parameter map, and the drive isolated from power as its own procedure requires.
Gather these before you touch the drive:
- Shielded twisted pair for the bus; three conductors carry the link, one per signal.
- An RJ45 plug crimped to the drive's socket wiring, which the next section gives.
- The panel manual's serial-port page: panels put RS-485 on screw terminals or on a second RJ45 with their own numbering.
- The drive manual's communications chapter, which prints the socket wiring, the serial settings and the parameter map you will address.
- The panel programming software with a Modbus RTU master driver for the serial port.
An RJ45 plug is just an eight-conductor connector. On the drive family this article uses as its example, the socket carries two-wire EIA-485 and sits under a small flap on the front, below the keypad slot. Isolate the drive and follow its manual's discharge procedure before you open that flap, because the internal capacitors hold charge after power off. For the broader picture of linking a panel to a controller, Connect a PC HMI to a PLC over a serial protocol or Ethernet covers the serial-versus-Ethernet basics; this article stays on the drive's pins, settings and registers.
Which pins on the VFD's RJ45 socket carry RS-485?
Looking into the drive's RJ45 socket with pin 1 at the top and pin 8 at the bottom, the RS-485 signals sit on pin 2 (RXTX+), pin 3 (common) and pin 7 (RXTX-), and the port runs two-wire EIA-485.
Number the pins from the socket side, into the drive, never from the cable side. Wire the panel's positive RS-485 line to pin 2, the negative line to pin 7 and the signal common to pin 3. Panel manuals label the two signal wires A and B, or + and -, and those labels follow no single industry convention. Match by polarity and let each manual's communications chapter confirm its own labels. Ignore an Ethernet patch cable's color code as a pin guide: the drive's serial port shares the plug shape with Ethernet and nothing else.
When every setting checks out and the master still gets silence, swap the two signal wires; reversed polarity on a two-wire bus is a 30-second test. The fuller sequence lives in Modbus RTU master no reply and address errors, which orders the checks for a master that gets no reply at all.
What serial settings does a VFD need for Modbus RTU?
Set both ends to 19200 baud, no parity, 8 data bits and 2 stop bits, written 19200,N,8,2. The example drive documents those values for its Modbus RTU port, and the Modbus specification pairs no parity with two stop bits.
Read the live values on the drive's communications parameters first, because a previous integrator may have changed them, then set the panel project to match. A mismatch in any of the four values blocks valid frames with the wiring perfect. Match baud rate, parity, data bits and stop bits together.
Timing has its own number. Modbus RTU frames need a silent gap of 3.5 character times between them, and at this speed a character is 1 start + 8 data + 2 stop = 11 bits. One character takes 11 / 19200 = 0.573 ms, so the gap works out to 3.5 x 0.573 = 2.0 ms. Keep the panel's poll delay above 2 ms so consecutive frames stay separate; most panels poll in steps of hundreds of milliseconds, well clear of the limit. An 8-byte read request occupies the wire for 8 x 11 = 88 bits, or 88 / 19200 = 4.6 ms at this speed.
How do you turn a VFD parameter into a Modbus register address?
Build the address as 4MMII: a leading 4, then the two-digit menu number, then the two-digit item number. Menu 3, item 8 becomes register 40308, and an on/off parameter still reads as the number 0 or 1.
The convention maps the drive's parameter numbering straight into the Modbus holding-register range. Menu and item numbers pad to two digits each, and the leading 4 marks a holding register. Menu 3, item 8 is 40308; menu 12, item 5 would be 41205. A parameter that shows only on or off on the drive display is still one register you read as a number, 0 or 1; it has no separate bit address.
Some drivers ask for a zero-based protocol address instead of the five-digit reference. Modbus references are one-based, with reference 40001 standing for protocol address 0, so the example converts as 40308 - 40001 = 307, written 0x0133 in hex. Enter whichever form the driver's addressing field expects. When a known parameter reads back a wrong value, check the address format before shifting every tag in the project.
What does a worked example of the whole connection look like?
In this example a 6-inch touch panel reads one parameter from one drive across a three-wire RS-485 link, and the numbers run the socket wiring, 19200,N,8,2 settings, register 40308 and a first value that matches the drive's display.
Work it in this order:
- Isolate the drive, follow its manual's discharge procedure, then open the front flap below the keypad slot.
- Crimp the RJ45 with conductor A on pin 2, conductor B on pin 7 and conductor C on pin 3, using shielded twisted pair.
- At the panel, land conductor A on the terminal its manual marks as the positive RS-485 line, conductor B on the negative line, conductor C on signal common.
- With power still off, check continuity from drive pin 2 to the panel's positive terminal, pin 3 to common and pin 7 to the negative terminal.
- Set the panel's driver to Modbus RTU master, 19200 baud, no parity, 8 data bits, 2 stop bits, and enter the slave address the drive's communications parameter shows.
- Set the drive's communications parameters to the same 19200,N,8,2, power up and read the live values back from the keypad.
- Build the first tag on menu 3, item 8: reference 40308, or protocol address 307 (40308 - 40001 = 307, hex 0x0133) if the driver wants that form.
- Run the panel's communication test, put the tag on a screen and read it.
Then compare the number on the panel with the drive's own display for parameter 3.08. Same number: the wiring layer, the settings layer and the address layer have all passed at once, and the link is ready for real screens. Add write tags one at a time afterwards, starting with a parameter the process tolerates changing.
How do you check the panel-to-drive link is working?
The link is working when the panel's communication test passes and a read of register 40308 shows the same number the drive's own display shows for parameter 3.08. Prove the read path before any write goes on the link.
Run the three checks in this order:
- Communication test: the panel's own protocol test completes without a timeout.
- Read check: register 40308 returns, and its value equals the drive display.
- Write check: write to one parameter the process tolerates, then confirm the drive display changed to match.
Keep start, stop and speed commands off the link until the read check has passed and a harmless write has succeeded; a Modbus write can move a running drive. When a check fails, change one layer per attempt: wiring, then settings, then slave address, then address format. Two changes at once leave you with no idea which one fixed or broke the link.
Why does the ping test pass but the protocol test fail over Ethernet?
A ping tests the IP path and nothing above it. Four replies from 192.168.1.100 at 1 ms can sit next to a protocol test that times out, and the cause is driver selection, not the network: choose the Modicon Modbus TCP/IP Ethernet driver.
One real case: the panel project had the ECOM Ethernet driver for PLC modules selected, with the module-name field reading SM-Ethernet and the IP address set to 192.168.1.100. The panel's ping test returned replies of 32 bytes at 1 ms, TTL 60, and the protocol test timed out every time. The ECOM driver speaks the PLC vendor's protocol to an ECOM module inside a PLC; the drive's Ethernet interface expects Modbus TCP. The ping replies came from the network stack, which answers no matter which driver the project loaded, so they told you the cable and IP path were good and nothing about the protocol.
Identify what sits at the far end of the panel's cable before touching the project. An ECOM module in a PLC takes the ECOM driver. A direct connection to the drive's Ethernet module takes the Modicon Modbus TCP/IP Ethernet driver, pointed at the IP address that interface actually holds. Run the protocol test again, then read one known register. The same layer confusion appears outside drives too; HMI pings the PLC but reads fail walks the driver checks for the general case.
Which fault points at which layer of the link?
Match the symptom to one layer: a serial timeout points at wiring or settings, a passing ping with a failing protocol test points at the Ethernet driver, and a connection that works but shows a wrong value points at the register address.
| Symptom | Layer to suspect | First check |
|---|---|---|
| Serial protocol test times out | Wiring or serial settings | Continuity on pins 2, 3 and 7, then confirm 19200,N,8,2 on both ends |
| Value reads but looks wrong | Register address | Recheck the menu and item numbers and the driver's address format against one known parameter |
| Read works, write has no effect | Parameter protection or write function | Confirm the parameter accepts writes and which write function the driver's documentation specifies |
| Ping replies, protocol test fails | Ethernet driver selection | Swap to the Modicon Modbus TCP/IP Ethernet driver and target the drive interface's own IP |
Each layer has its own pass signal, which is why a stubborn fault usually means two layers were changed at once. Fix one, re-run the matching check, and move on only when it passes.
Recommendation: Use the serial path for a panel-to-drive link unless the drive already carries its Ethernet module. Use three conductors on pins 2, 3 and 7, 19200,N,8,2 on both ends, the first tag on register 40308 as a read-only test, and no write commands until that read matches the drive display. On the Ethernet path, select the Modicon Modbus TCP/IP Ethernet driver and point it at the drive interface's own IP address, never at the ECOM PLC driver.
The panel is the machine's operator interface and stays part of the control system. When the cell also needs a standalone board for shift notes, quality counts or downtime status, that is a signage job, and our panel-mount displays are built for that kind of offline board.
Questions
Can I use a straight-through Ethernet patch cable between the panel and the drive?
A straight-through patch cable passes every pin one to one, so pin 2 reaches pin 2 and pin 7 reaches pin 7. That only works if the panel end terminates those same pins to its RS-485 signals. Most panels put RS-485 on screw terminals or use their own RJ45 numbering, so build the cable to the drive's map (RXTX+ on pin 2, common on pin 3, RXTX- on pin 7) and check continuity end to end before power-up.
How many drives can one panel talk to on the same RS-485 bus?
A Modbus RTU master addresses slaves numbered 1 to 247, and a classic EIA-485 segment carries up to 32 unit loads without repeaters. The drive manual's transceiver loading figure sets the real number. Wire the bus as a daisy chain, keep the drop to each drive short, terminate both ends (120 ohms is the standard value for typical twisted-pair data cabling) and give every drive a unique address.
The panel only has an RS-232 port. Can it still connect?
Not directly. The drive's socket is two-wire EIA-485, and RS-232 uses different voltage levels and single-ended signalling, so put an RS-232-to-RS-485 converter between them. Choose one that handles the transmit-to-receive turnaround automatically. The converter's RS-485 side then wires to pins 2, 3 and 7 exactly as a native RS-485 panel port would.
How long can the cable run be at 19200 baud?
At data rates like 19200 baud, RS-485 runs on twisted pair are routinely planned up to about 1200 m, roughly 4000 ft, with termination at both ends. At this speed the limit comes from the cable's attenuation and reflections more than from the data rate, so keep drop lines short and avoid spur wiring hanging off the main run.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.