S7-200 Freeport

[rajsiyer 7]

Hi fellas, I've a new problem for which I need some advice. At present there is an S-7 cpu224XP with its freeport configured as MODBUS and through which it reads data from 4 temperature scanners (each having its own Modbus RS485 serial port). There is no HMI/Scada at present and the PLC reads the temperature data from the 4 scanners and uses the same in its logic. Now, my task is to connect this arrangement to a PC with an VB application as SCADA. I propose to use a Modbus DDE server for the VB application as I'm familiar with it. How does one go about the problem? My apprehension is...Once the PC becomes the master, will the PLC be able to poll data in the temperature scanners? . If let's say I configure the PLC's freeport as Modbus slave( and assuming that it works!) it will yield data to the PC in the same way as the 4 scanners.So the temperature data will have to be written by SCADA into the PLC. This in turn means that with the SCADA PC off-line, the PLC logic will not work as it will not get the temperature data. This is therefore an unsafe option!. So the question begs is which is the best way to do it?? With warm regards & best wishes Raj S. Iyer

[BobLfoot 302]

You asked for $0.02 so here goes. In your shoes I'd try running my PC as a slave having the PLC read data from it and send data to it on a cyclic pace. This might not be the most efficient setup, but it disrupts the current layout the least and makes the PC an auxiallary device not a mandatory one.

[rajsiyer 7]

Hi Bob; Solved it today. Matter was simpler than I had worried.(Temperature readings were wired to the AI of PLC, and not through Modbus) However as I learnt a few things about the S7-200's abilities, must share it here. Thanks for your Interest & prompt reply. I reciprocate. 1. The S7200's "freeport" is its com0 port and in Modbus it cannot be configured as a master. It can only yield data as Modbus rtu. 2. For configuring the PLC's port a s Modbus RTU, you need to purchase and install the Library of Modbus functions. It is sold as a separate CD which contains various Modbus and USX protocol functions. Without this library, it is not so easy to configure the Modbus port functions. The function is called "Modbus_init" and it appears as a little function box in the ladder editor wherein you parametrise the line parameters such as baudrate and Slave ID, Parity etc. 3. The Modbus like behaviour of the port is only at run-time. When PLC is stopped the port reverts back to the PPI mode to enable transfer of program etc. 4. I used a ordinary ADAM-4520/MOXA 485-232 convertor at the PC's com port and using MODSCAN, was easily able to see live data streaming from the PLC as well as the temperature controllers. Then I used Wonderware's DDE server for Modbus. It works like a cinch. Live data from all slaves were readily transferred to EXCEL. Regards.. Raj S. Iyer