MSG — Modbus RTU

---

Description

With the protocol set to modbus-rtu, MSG reads or writes registers or bits in a Modbus RTU slave over a serial line: an RS-485 multidrop or an RS-232 point-to-point link to a small PLC, a drive, a power meter or a remote-I/O block. The Arduino is the master; the rung’s rising edge queues a request and a master service runs the bus a little on every scan, one bus per serial port, so the logic never waits. Areas, marshalling and the status tag are the same as Modbus TCP; only the transport differs. Use it wherever a slave has a serial port and a register map. Do not put two masters on one bus, and do not expect it to work through the primary Serial; that port is the Get Online link.

---

Operands

Operand	Type	Format	Valid Range	Required	Description
Direction	Choice	read / write	Write is refused on Input Register and Discrete Input areas	Yes
Protocol	Choice	modbus-rtu	Selected for you when the device is a Modbus RTU Device	Yes
Port	Comm port	Serial1, Serial2, Serial3 or SoftwareSerial	A UART port; not Serial	Yes	Every RTU device on one port shares one master and one bus.
Device	Network device	Name	A Modbus RTU Device on that port	Yes	Carries the slave’s unit id (1 to 247), baud, parity, the SoftwareSerial RX/TX pins, an optional DE pin and the 32-bit word order.
Local data	Tag	INT, SINT, DINT or REAL array for registers; BOOL array for coils / discrete inputs; or Buf[n]	Window must fit	Yes	INT/SINT = 1 register, DINT/REAL = 2, BOOL = 1 bit.
Length	INT	1 up to one frame (125 registers, 1968 bits)	Build error above the limit	Yes	Elements of Local.
Start address	INT	0 to 65535		Yes	0-based protocol address.
Modbus area	Choice	Holding Register, Input Register, Coil, Discrete Input		Yes
Status (opt)	CONTROL tag	Name	One per MSG	No	EN, DN, ER, POS = exception code; 255 = timeout, CRC error, wrong unit or function in the reply.

---

Scan Behavior

Prescan

Nothing. The serial port is opened in setup() at the device’s baud; 8N1 on SoftwareSerial, the device’s parity on a hardware UART.

Rung-condition-in is false

Nothing new. A queued or in-flight request continues. The edge memory is armed.

Rung-condition-in is true

On the rising edge the request is queued; with a status tag, EN = 1, DN = ER = 0. The master service takes requests one at a time per bus: it raises the DE pin if one is configured, sends unit + function + data + CRC-16, releases the line, and collects the reply until the expected length has arrived or the line has been quiet for the 3.5-character gap. CRC, unit id and function code are checked. Success unpacks a read into Local and sets DN; an exception sets ER with the code in POS; no reply within one second sets ER with POS = 255. Function codes are those of the Modbus TCP page: FC03/16 holding, FC04 input, FC01/05/15 coils, FC02 discrete inputs.

Postscan

Nothing.

---

Status Tag Members

Member	Data type	Set by	Cleared by	Description
EN	BOOL	Rung edge	Service on completion	Queued or in flight.
DN	BOOL	Service on success	Next rung edge	Last transfer succeeded.
ER	BOOL	Service on failure	Next rung edge	Last transfer failed.
POS	DINT	Service	—	Exception code (1 to 4 as in Modbus TCP); 255 = timeout, CRC, unit or function mismatch.

---

Example

Scenario: An AutomationDirect CLICK PLC on an RS-485 line. Its first two data registers DS1 and DS2 sit at holding-register address 0 and 1. A pushbutton reads them.

Network tree: Click — Modbus RTU Device on the SoftwareSerial port, unit id 1, 9600 baud, no parity, RX D2, TX D3, no DE pin (the transceiver switches direction itself), word order CDAB.

Tags:

• Read_Click — Read CLICK registers, BOOL
• Click_Regs — CLICK DS registers, INT[4]
• Click_Sts — CLICK message status, CONTROL

Rung 1:

—|XIC Read_Click|———[MSG  READ MODBUS-RTU  Device Click  Local Click_Regs  Length 2  Holding 0  Status Click_Sts]———

Scan 1 — Read_Click = 0. Nothing queued.

Scan 2 — Read_Click = 1 (rising edge). The request is queued, Click_Sts.EN = 1. On hardware 01 03 00 00 00 02 C4 0B goes out, the CLICK answers with two registers, Click_Regs[0] and [1] take DS1 and DS2, and Click_Sts.DN = 1.

Values before and after: Click_Regs[0..1] 0,0 → DS1, DS2; Click_Sts.DN 0 → 1. In the simulator nothing is sent.

---

See Also

• MSG — the instruction and its dialog
• MSG — Modbus TCP — the same areas, function codes and binary layout over Ethernet
• MSG — I2C, SPI and UART — raw serial without framing
• Adding Network Devices — the RTU device, its pins and word order
• Communication Instructions — Category index

---

Notes

One master per bus, many slaves. Put every slave on the same RS-485 line under the same port; they differ only by unit id. The build refuses two devices on one port that disagree on baud, parity or pins, a duplicate unit id, or a unit id outside 1 to 247.

SoftwareSerial is 8N1. Parity is only available on a hardware UART. On an Uno set the slave to no parity, 9600 or 19200 baud, and give the device two spare pins; the project may have only one SoftwareSerial link, so a DF1 device cannot share the board.

DE pin. A bare MAX485 needs its driver-enable pin driven; name it on the device. Shields with automatic direction control leave it blank.

Addresses and word order follow the Modbus TCP page: 0-based, vendor offsets removed, DINT/REAL word order set on the device. The CLICK, for instance, documents its DS registers at 400001 and up, which is address 0, and swaps words (CDAB).

Verified against a CLICK C2-03CPU over RS-485 with an Uno and the RS-232/RS-485 shield in its SoftwareSerial mode on D2/D3.