MBRTU_Read

Read a single register from a Modbus/RTU device

Description

MBRTU_Read(unitid, cmd, reg, format=BT_UINT16, &error)

This command issues a Modbus/RTU read command and waits for a valid reply. If an error occurrs (timeout, or error reply from the device), the error parameter is set to a non-zero value.

Using this command in the user script will disable any system drivers using Modbus to avoid conflicts.

Parameters

unitid Modbus/RTU device bus address (1-254)
cmd The Modbus command, see below
reg The register physical address
format Register type, see below. Optional, defaults to UINT16
error Variable to receive the result code. Optional.

Modbus commands

MB_COIL Read coil (Modbus command 1)
MB_DISCRETE Read discrete input (Modbus command 2)
MB_HOLDING Read holding register (Modbus command 3)
MB_INPUT Read input register (Modbus command 4)

Register types

BT_INT8 8 bit integer, signed
BT_UINT8 8 bit integer, unsigned
BT_INT16 16 bit integer, signed
BT_INT16LE 16 bit integer, signed, byte swapped
BT_UINT16 16 bit integer, unsigned
BT_UINT16LE 16 bit integer, unsigned, byte swapped
BT_INT32 32 bit integer, signed (2 Modbus registers)
BT_UINT32 32 bit integer, unsigned* (2 Modbus registers)
BT_INT32LE 32 bit integer, signed, word flipped (2 Modbus registers)
BT_UINT32LE 32 bit integer, unsigned*, word flipped (2 Modbus registers)
BT_INT32BLE 32 bit integer, signed, byte flipped (2 Modbus registers)
BT_UINT32BLE 32 bit integer, unigned*, byte flipped (2 Modbus registers)
BT_INT32LBE 32 bit integer, signed, byte and word flipped (2 Modbus registers)
BT_UINT32LBE 32 bit integer, unigned, byte and word flipped (2 Modbus registers)
BT_FLOAT IEEE 754 Floating point (2 Modbus registers). See below.
BT_FLOATLE IEEE 754 Floating point, word flipped (2 Modbus registers). See below.
BT_FLOAT64 IEEE 754 64 bit Floating point* (4 Modbus registers). See below.
BT_FLOAT64LE IEEE 754 64 bit Floating point*, word reversed (4 Modbus registers). See below.

Return value

The function returns the decoded value as a Pawn cell.

*Note that the return value is always a 32 bit cell. This means that a large 32 bit unsigned value will be represented as a negative number in Pawn.

Floating point return values must be tagged as Float, otherwise they are rounded to an integer.

64 bit floating point values will be converted to the closest 32 bit value.

If there is an error, the return value will be zero, and the error parameter will be set to a non-zero value.

Example usage

   new err, x;
   x = MBRTU_Read( 5, MB_HOLDING, 101, BT_INT16, err );
   if(err == 0) {
     // x holds the value of holding register 40102 (logical addressing)
   }

Example usage with floating point

   new err, Float:x;
   x = Float:MBRTU_Read( 5, MB_HOLDING, 201, BT_FLOAT, err );
   if(err == 0) {
     // x holds the Float-value of holding registers 40202,40203 (logical addressing)
   }

Modbus communication is non-trivial. Please refer to the Modbus documentation at https://modbus.org. eze System can't assist with questions related to third party devices. We strongly recommend using our developed and tested drivers for any deployed systems. The functions described here should only be used for testing.