1756-DHRIO connection to DCS

[gromit 4]

I need to set up a 1756-DHRIO connection to a DCS via DH+ protocol. Do I need to incorporate messages, or does the DCS have access to the data in the PLC memory locations? Also, is there a difference between a 1756-DHRIO/B and 1756-DHRIO/C module?

[Ken Roach 194]

Generally, DCS gateways to A-B controller have an RS232 serial link that connects to a DF1/DH+ bridge like the 1770-KF2 or 1785-KE module. The DCS polls the interface module, which in turn requests data tables from the controller. In most of the systems I have seen, the ControlLogix is configured to emulate PLC-5 type data tables. You need to ask your DCS vendor to describe their Allen-Bradley interface. The Series C version of the 1756-DHRIO has different hardware than the Series B version, and each has firmware specific to its hardware. Do not load Series A/B firmware onto a Series C module, and vice versa. In terms of performance, Series B and C are essentially identical. Series D added the ability to run one channel at 230.4 kb/s with DH+ protocol.

[Ken Moore 23]

What Ken Roach said was generally true until about 2-3 years ago. Many DCS vendors now have dedicated comm. modules for most of the industrial protocols. I have an older DCS using the KF2, but I plan on upgrading next year to a dedicated DH+ module. Regardless of the hardware link, the DCS will poll the plc for the required data, the interface is similar to a SCADA system interfacing with a PLC via DH+, no messaging required.

[gromit 4]

Thanks! If I understand correctly, I should not need additional hardware, such as the KF2 module or other. The DCS should be able to connect directly to the DHRIO serial port. Also, I believe that I understand that the DCS will reach out and grab the desired registers from the ControlLogix memory, without me having to message them out or put in special memory areas. Please confirm.

[Ken Moore 23]

Depends on the DCS, not all of them are able to read CLX memory unless you map it to PLC-5 type data tables.

[Ken Roach 194]

Gromit, it would be a really big help if you posted what make and model of DCS control system you plan to use. The 1756-DHRIO doesn't have a serial port. This is "putting the data in special memory areas", in my opinion. It's not hard, but you do need to plan and structure your data exchange.