MSG — DF1

Category: Communication
Type: Output
Availability: All platforms; a hardware serial port (Serial1/2/3, Mega recommended) or the SoftwareSerial port, plus an RS-232 level converter

MSG instruction block for a DF1 write: Type: WRITE DF1, Device: Micro, Local: DataToMicro with its value [99, 6], Length: 2

Description

With the protocol set to df1, MSG reads or writes an Integer or Float data file in an Allen-Bradley controller over its DF1 serial port: a MicroLogix, an SLC 500, or a CompactLogix Channel 0 with a PLC/SLC message mapping. The Arduino is the DF1 master and speaks PCCC full-duplex with CRC, the same wire protocol RSLinx uses on a serial driver. The rung’s rising edge queues the request; a shared service runs the link on every scan. Use it where the PLC has a serial port and no Ethernet, or where a spare Channel 0 is the easiest way in. Do not run it on the primary Serial; that suppresses the Get Online monitor.

Operands

Operand	Type	Format	Valid Range	Required	Description
Direction	Choice	`read` / `write`	—	Yes	Typed Read (0xA2) or Typed Write (0xAA) of the remote file.
Protocol	Choice	`df1`	Selected for you when the device is a DF1 PLC	Yes
Port	Comm port	`Serial1`, `Serial2`, `Serial3` or `SoftwareSerial`	Not `Serial`	Yes	The transport follows the tree: a hardware UART is full-duplex and leaves USB free; SoftwareSerial uses the RX/TX pins set on the device.
Device	Network device	Name	A CompactLogix (DF1 / CH0) or SLC 500 / MicroLogix (DF1) device on that port	Yes	Carries baud, node numbers (DST = PLC, SRC = Arduino), the SoftwareSerial pins and the debug switch. All DF1 MSGs share this one link.
Local data	Tag	INT array for an Integer file, REAL array for a Float file; or `Buf[n]`	Window must fit	Yes	The file type is tied to this tag’s type.
Length	INT	1 to 128	Fits the array	Yes	Elements (words for N, floats for F).
Remote file type	Choice	Integer (N) or Float (F)	Must match the Local type	Yes	Follows the Local tag automatically; a mismatch is a build error.
Remote file number	INT	0 to 255	The PLC’s file, e.g. 7 for N7	Yes
Remote element	INT	0 to 999	Start element, e.g. 0 for N7:0	Yes
Status (opt)	CONTROL tag	Name		No	Accepted but not filled by DF1 in this release; the members stay 0.

Scan Behavior

Prescan

Nothing. The serial port opens in setup() at the device’s baud.

Rung-condition-in is false

Nothing new. A queued request continues. The edge memory is armed.

Rung-condition-in is true

On the rising edge the request is queued. The DF1 service, round-robining every DF1 MSG on the link, sends the PCCC command (Typed Read or Typed Write of N<file>:<element> or F<file>:<element>, Length elements), handles the DF1 ACK/NAK/ENQ handshake and CRC, and on a read copies the reply into Local. A NAK or a missing reply is retried up to three times, then the request is dropped. There is no status tag feedback in this release, so watch the data itself, or the device’s debug output on USB, to confirm traffic.

Postscan

Nothing.

Example

Scenario: A Mega talks to a MicroLogix over Serial1. Every fast pulse reads two words from N7:0-1; every medium pulse writes two words to N7:2-3. This is the MEGA.ldp example project, routine R001_Ctrl.

Network tree: Micro — CompactLogix (DF1 / CH0) device on Serial1, 19200 baud, DST node 0, SRC node 1.

Tags:

Pulse.Fast, Pulse.Med — pulse bits from a flip-flop block
MicroData — INT[2], read from the PLC
DataToMicro — INT[2], preset [99, 6], written to the PLC

Rung 0 (read):

—|XIC Pulse.Fast|———[MSG  READ DF1  Device Micro  Local MicroData  Length 2  N7:0]———

Rung 1 (write):

—|XIC Pulse.Med|———[MSG  WRITE DF1  Device Micro  Local DataToMicro  Length 2  N7:2]———

The figures follow the write rung; the read rung is the same block with Direction read, Local MicroData and element 0.

Figure 1 — The MSG dialog for the DF1 write: write, df1, port Serial1 on D19/D18, device Micro (CompactLogix DF1 / CH0), local DataToMicro, length 2, file type Integer (N), file 7, element 2, no status tag; the path line reads DataToMicro to N7

Write rung, pulse off. Nothing queued. The face lists DataToMicro as [99, 6].

Figure 2 — DF1 MSG write rung false: Pulse.Med off, wire dark, the block showing WRITE DF1, Micro, DataToMicro [99, 6], Length 2

Write rung, pulse on. The write is queued; on hardware N7:2 and N7:3 become 99 and 6.

Figure 3 — DF1 MSG write rung true: Pulse.Med on, the wire lit through the block showing WRITE DF1, Micro, DataToMicro [99, 6], Length 2

Values before and after: N7:2-3 → 99, 6; on the read rung MicroData 0,0 → N7:0-1. In the simulator nothing is sent; the pulses still blink, so a frame catches the rung in either state.

Notes

A level converter is mandatory. The PLC’s RS-232 swings ±12 V; an Arduino pin does not survive it. Put a MAX3232-class board between them. Wiring, PLC-side setup and the CompactLogix message mapping are in the DF1 setup guide shipped with the IDE.

File type follows the Local tag. INT and DINT locals read Integer (N) files, two bytes per element; REAL locals read Float (F) files, four bytes per element. The dialog syncs the dropdown when you pick the Local tag; leave it that way unless you mean to read a float file into integers.

CompactLogix needs a mapping. A Logix controller has no N7 until you map a tag to file 7 in the controller’s PLC/SLC Message settings. The MicroLogix and SLC already have their files.

One link per project. All DF1 MSGs share the device’s port. On an Uno that is the SoftwareSerial port, which a Modbus RTU device cannot also use; a Mega with its spare UARTs is the recommended board.

Debug. The device’s Debug switch prints each transaction’s verdict on the USB serial and suppresses the monitor while it is on.

Verified in both directions against a MicroLogix 1100 on a Mega’s Serial1.

Applies to LadderIDE >=1.2.2 · Last reviewed 2026-09-11 · Screenshots verified 2026-09-11