We carried out a series of tests using Modbus poll software running on a PC to communicate with F2424. We found that Modbus poll can send Modbus function 01, 02, 03, 05, and 06 to the F2424 for tens of thousands of messages without a single failure. But when we ran Modbus function 16 test (multiple word write) on the F2424, we found that indeed 1 in approximately 250 commands sent resulted in a no response. It doesn't matter if we select a polling period of 1ms, 10ms or 100ms. The statistics is similar.
It appears that there may be a bug or imperfection in the firmware code that handles the Modbus function 16. This will be reported to the firmware development team for a fix for future firmware release.
For your existing application, you may be able to tolerate the occasional missing comm reply by reducing the time out period. The PLC would normally reply well within less than 100ms. So you can choose a time out of 100ms and the disruption should be brief and the DAQfactory software can resend the Modbus write command.