Applies To:
  • CitectSCADA 6.00

I am using the S7NT protocol and I want to use the maximum block size for the driver. In Citect Help it says that the maximum request length is 2048 bits (256 bytes) for the driver but in the protdir.dbf the max_length and the block_bit fields are both set to 1776 bits. Why?  

The reason why the max_length and the bit_block fields have been set to 1776 bits is because the S7-300 PLC can accept request lengths of up to 222 bytes when used with the S7NT protocol. However, the S7-400 PLC can accept request lengths of up to 2048 bits(256 bytes) from Citect.

So if you are using a system where ONLY the S7-400 PLC is used with the S7NT driver, then in order to boost the performance of your system, you can change the maximum request length and maximum block size of the S7NT driver from 1776 to 2048 bits in the protdir.dbf file.

However if your system consists of both the S7-400 and S7-300 PLCs or consisting of only the S7-300 PLC being used over the S7NT driver, then you should leave the maximum request length and the block size to 1776 bits in the protdir.dbf as the S7-300 PLC does not accept blocks of more than 222 bytes.

Citect has confirmed this to be a problem in CitectHMI/SCADA version V6.00. We are researching this problem and will post new information here as it becomes available.