Connecting Modbus slave to S7-400


Thread Starter

Amir Or

Hi list.
Has any one used the CP-441-2 to communicate with Modbus slaves ?
What are the limitations ?
Can I use 3 CPs in the main rack (I need 5 ports )?

Daniel Chartier

We have recently setup the same configuration you are looking at: 3 CP441-2 with 5 Modbus slave ports. We used the Siemens examples to test each connection individually and then built a FC1000 that synchronized all the Modbus FC calls to the slaves. It consisted of 8 x 0,5 sec. triggers, each one calling simultaneous Modbus functions on all slaves. That way we divide the workload on consecutive calls.
It took some bit of work, but has been working seamlessly for the past 4 months.
What you need to look out for: first, serial connections must be carefully executed; any error on the soldering wires (missing ground or a short between wires) will result in a lack of exchanges between master and slave, with no diagnostics. Second, you must test the limits of your slave in multiple-register operations: for Example, Modbus FC16 is a multiple Write; some of our slaves would exchange up to 12 registers, others were limited to 6; again no diagnostics, just failure. Third, addressing schemes are probably different between the Siemens master and the Modbus slaves, especially if they are from other PLC manufacturers, so you might get data transfered, but it might be undecipherable. Look out for byte inversions (registers that send the LSB before the MSB); also sometimes you must request a start register of 40001 (for ex.) in order to get 40000.
Hope this helps,
Daniel Chartier