Modbus Master Protocol

The DataLink DL2000-CMX contains both Modbus Master and Modbus Slave protocols. When selected for Master mode the DataLink DL2000 is a Modbus network "Master". It enables PLCs (or smart devices such as PCs) on an Allen-Bradley Data Highway Plus network to initiate Read/Write commands to Modbus networks from one to multiple slave devices. The DataLink provides both RS232 and RS422/RS485 serial interface connections to Modbus.

The DL2000-CMX (Master) is a Modbus Master and responds to commands from any PLC-5 on the DH+ network, translates them to Modbus format and transmits them to any Modbus slave device on the Modbus network. A Modicon BM85 would allow PLC-5 commands to be sent to Modicon PLCs on Modbus Plus.

Communications between Modbus and the DataLink are initiated from a station on DH+. The DataLink must be the only Master on the Modbus network and does not require a Modbus address. The DH+ network side of the DataLink unit is an active station on the DH+ and requires a unique DH+ address. DataLinks will automatically accept either PLC-5 Typed Read/Write commands or PLC-3 Word Range Read/Write communications from a PLC or SLC (on DH+) message instruction or from intelligent devices on DH+ such as PCs running HMI/GUI software.

The DataLink uses the Destination Data Table address format N xxx . yyy used in all DH+ message instructions as a Modbus address. Here, the N represents Register commands, xxx the slave station number (1-255 decimal) and yyy the last 4 digits of the register address (1 to 6666). Its register address capability is dependent on the PLC type, DH+ message command set and PLC programming software used. SLC-5/04s are limited to 255, PLC-3s to 9999, and PLC-5s to 9999 (see below).

Addressing:

The Modbus register range addressable by PLC-5s using PLC-5 message commands are:

Operation:

The DataLink receives a command on its DH+ network channel, converts it to the appropriate Modbus command, and sends it out on the asynchronous RS232C Modbus channel to the addressed Modbus slave device. The Modbus slave response is received by the DataLink Master, converted into the corresponding DH+ response and directed back to the originating DH+ station.

When the DataLink receives a PLC-3 Word Range Read/Write or a PLC-5 Typed Read/Write message command addressed to its DH+ station number, it automatically interprets it. In response to DH+ Read messages it generates a Modbus 03 command, and generates a Modbus 16 command to DH+ Writes.