Master Slave Protocol

The DataLink DL2000-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/rite messages to any station on Allen-
Bradley's Data Highway Plus. The DataLink provides both RS232 and RS422/RS485 serial
interfaces to Modbus.

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.

The DataLink supports Modbus commands: 01, 02, 03,
04, 05, 06, and 16 and Broadcast on commands 05, 06
and 16 . One Modbus command can initiate a read or write
transfer of up to 100 registers or 1600 coils.

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 (Bit) data
together with an address translation map from Modbus to A-B for up to 1000 words to
16,000 bits.

Operation:

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
address is an active address on DH+. If there is an address match the DH+ station will ACK
the Modbus command translate it to A-B DH+ and send it to the appropriate DH+ station. If
the Modbus command was a write then the Modbus sequence is complete, however, a
Modbus read command is not complete until the read data is received.

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.