Mixing Modbus and Rockwell on Ethernet

Both Modbus and PCCC-based protocols like DF1 or CSPv4 (AB/Ethernet) have been around for years. Yet if one looks at the similarities between the two, one quickly sees that the act of reading 10 words from an N7 data file is exactly the same as reading 10 words from Modbus 4x00001. The Digi One IAP leveraged this to become the world's first off-the-shelf transparent protocol bridge. It freely accepts Modbus or Rockwell requests and bridges them to the appropriate form for the slave to understand.

Here is an example system:

• The ControlLogix can poll the Modbus/TCP and DF1 PLC
• The Modbus/TCP PLC can poll the ControlLogix and DF1 PLC
• The DF1 PLC can poll the ControlLogix and Modbus/TCP PLC.

So how does this work? Take a look at the messages to read the first 48 bits of bit memory:

• Modbus/TCP is 001E00000006010100000030
• Modbus/RTU is 0101000000303C1E
• Modbus/ASCII is :010100000030CE(CR)(NL)
• DF1 Full-Duplex is 100201000F000019A206038500001003DE06
• CSPv4 is 0107000E00 … 010500000F000019A2060385000
• PCCC-Ethernet/IP is 6F002800 … 0000010000000F000019A2060385000

Notice the bold, underlined text patterns? This is the heart of how a normal Modbus Bridge or 1761NetENI function. Modbus/TCP, Modbus/RTU, and Modbus/ASCII may have different bytes, but they all move the exact same Modbus command; a Modbus bridge doesn't need to understand the Modbus command, just be able to unpack and repack each form. Similarly DF1, CSPv4, and PCCC-in-Ethernet/IP have different bytes, but they all move the same PCCC command; a PCCC bridge doesn't need to understand the PCCC command, just be able to unpack and repack each form.

The Digi One IAP takes this one step further - since each of these bold, underlines commands is accomplishing the same thing - namely reading the first 48-bits of bit memory - the Digi One IAP can take either command and mechanically create the other. So given the Modbus command 010100000030, it can create the PCCC command 0F000019A20603850000. Given the core PCCC command 0F000019A20603850000 it can create the Modbus command 010100000030. So this how a Modbus/TCP master can query a ControlLogix with PCCC-enabled. the Modbus/TCP master thinks it is polling another Modbus device. The ControlLogix thinks it is being polled by another Ethernet/IP device.

Here are links to other related information:

Digi One IAP product page
Application Note for Modbus master polling Rockwell devices.
Excel spreadsheet for Modbus master polling Rockwell devices.
Application Note for Rockwell master polling Modbus devices.
Excel spreadsheet for Rockwell master polling Modbus devices.
PDF presentation of various ways to mix Modbus and Rockwell devices