Applies To:
  • CitectSCADA 5.xx, 6.XX
  • CitectHMI 5.xx, 6.XX

When using Industrial Ethernet to access some I/O on a remote rack, the following error messages were logged in the syslog.dat file:

Driver S7NT on port Port1 reports 'request size too big' error.
The compiled request size is too big for the driver. This normally indicates a problem with the protocol driver.
Please contact Citect Support or the driver developer.

RawType 0, UnitType 4, UnitAddress 32, UnitCount 696.

Mon Nov 28 03:02:00 2005 00:29:54.661 Error: General error
READ 0008 Port1 PLC PEINT264,40 80
Generic 000008 Driver 00008711 (0x00002207)

The variable tags displayed on screen would intermittently go #COM, then display their correct value.

What is wrong, and how do you resolve this issue?

This problem occurred because of the limitation on the number of I/O you can poll in a single request, and the increase of the maximum request size of the S7NT driver.
In the S7NT driver version, the maximum request size was increased from 512 to 1776. This meant that on certain networks (i.e. Profibus and Remote I/O Racks) the maximum request size would be exceeded. The solution is to use the latest S7NT driver, which has reverted back to the maximum request size of 512. But only use the S7NT driver for variable tags which are accessed via profibus or remote I/O. If you are polling Data Blocks directly from an S7-300 or S7-400 CPU via Industrial Ethernet, you can use protocol S7NT300 and S7NT400 respectively, to take advantage of the increased request size these PLCs support.  For further information, see KB Q4390.

Citect has confirmed this to be a problem in S7NT driver versions to This problem has been fixed in version

Request size too big, Profibus, remote I/O, S7NT, S7