DL3000-CMX:
DH+ to Modbus Slave Protocol
 

The DL3000-CMX contains both Modbus Master and Modbus Slave protocols. When configured for Slave mode the DataLink is a "Slave" on a Modbus network and enables a Modbus Master to initiate PLC-5 Typed Read / Write messages to any station on Allen-Bradley's Data Highway Plus. The DataLink provides an RS232 serial interface to Modbus and optional RS422/RS485 interfaces are available at additional cost. In Slave mode communications between Modbus and DH+ can only be initiated by the Modbus Master. The Modbus serial port on the DataLink is transparent to Modbus commands and does not require a Modbus slave address. The DH+ network side of the DataLink is an active station on the DH+ and requires its own unique DH+ address. Any DH+ station (except 0) capable of responding to PLC-5 Typed Read/Write commands, can receive commands from the Modbus Master andrespond as if they were slave Modicon PLCs. Station 0 is also a valid address for the Slave DataLink.

Addressing:
A Modbus to DH+ data interchange table can be configured in the DataLink to provide 8 address range entries. Configuration permits both Register (Word) and Coil (Discrete / Bit) data together with an address translation map from Modbus to A-B for up to 1000 words or 16,000 bits.
 

The Modbus Master issues commands to slave addresses which may be on either a Modbus Slave network or on the DH+ via the DataLink. The DataLink monitors all commands and addresses on the Modbus Slave network and checks to see if the Modbus Slave address is the same as an active address on DH+. If there is an address match the DL3000-CMX (Slave) will translate the Modbus command to an A-B DH+ message and send it to the appropriate DH+ station. If the Modbus command was a write then the Modbus sequence is complete and an appropriate write response can be immediately returned to the Modbus Master; however, if the Modbus command was a read, the destination DH+ station has to gather the requested data and return it to the DL3000-CMX before a correct Modbus Slave response and the data can be returned to the Modbus Master.

A Modbus Master can read and write to A-B PLC-5s on DH+ without the need for any programming in the PLC, however the PLC addresses used must not be in "protected mode" and the full range of addresses which the Modbus Master requires to read or write must have been created in the PLC's Data Table.