Read Data

Description

A read operation transfers data from a specified slave source device to a master destination device on the network. The operation uses a master transaction path, and may require several cycles to complete. To program an MBP_MSTR block to perform a write operation, use function code 2.

NOTE: Do not attempt to program an MBP_MSTR to read to its own station address. This action causes the function block to generate an error in the CONTROL[2] register of the control block.

You can perform a read operation on a nonexistent slave register. The slave detects and logs the status, which can last for several cycles.

NOTE: For Quantum PLC communication with a Premium/Atrium PLC, confirm that the addressing is made with an offset of 1. In order to access an address object n of a Premium PLC, confirm that the communication function of the Quantum PLC uses the n+1 address. IEC addressing starts at 0, and Modbus addressing starts at 1.

Network Implementation

The read operation can be performed on Modbus Plus, TCP/IP Ethernet, and SY/MAX Ethernet networks.

Control Block Usage for Modbus Plus

Register	Meaning
CONTROL[1]	2 = read data
CONTROL[2]	Indicates the error status.
CONTROL[3]	Number of registers to be read from the slave.
CONTROL[4]	Determines the %MW starting register in the slave from which the data is read, e.g., 1 = %MW1, 49 = %MW49.
CONTROL[5] ... CONTROL[9]	Routing register 1 is used to specify the address (routing path address 1 of 5) of the node during a network transfer. The last byte in the routing path that is not 0 is the destination node.

Control Block Usage for TCP/IP Ethernet

Register	Meaning
CONTROL[1]	2 = read data
CONTROL[2]	Indicates the error status.
CONTROL[3]	Number of addresses to be read from the slave.
CONTROL[4]	Determines the %MW starting register in the slave from which the data is read, e.g., 1 = %MW1, 49 = %MW49.
CONTROL[5]	Routing register most significant byte: network adapter module slot least significant byte: MBP on Ethernet Transporter (MET) mapping index
CONTROL[6] ... CONTROL[9]	Each address contains 1 byte of the 32-bit IP address, where the MSB is in CONTROL[6] and the LSB is in CONTROL[9].

Control Block Usage for SY/MAX Ethernet

Register	Meaning
CONTROL[1]	2 = read data
CONTROL[2]	Indicates the error status.
CONTROL[3]	Number of addresses to be read from the slave.
CONTROL[4]	Determines the %MW starting register in the slave to which the data is written, e.g., 1 = %MW1, 49 = %MW49.
CONTROL[5]	Routing register: most significant byte: network adapter module slot least significant byte: destination drop number
CONTROL[6] ... CONTROL[9]	terminator: FF hex