I have an 8864 logger communicating Modbus with 2 devices. I don't know Modbus at all. However, my file appears to read/write from one device and then go on to the next. This doesn't always happen to the same device, either. When I have the issue with data values being zeros, I can unplug the Ethernet connection for a minute and plug it back in. This usually allows communication to commence. Sometimes it lasts for a minute or two and other times it can be for hours or days (weekends). It only happens once every couple of months. I think I should add that it goes Ethernet to fiber optic to Ethernet and back Ethernet to fiber optic to Ethernet again.