ControlLogix DF1 question

Periodically, the PLC (DF1 Half Duplex slave) will return a DLE EOT to a DLE ENQ stn BCC.
I've looked at the data with a comm. analyzer and everything in the request is valid. As a matter of fact, it is the same exact request that was sent hundreds of times before this one that received the EOT but the PLC responded with data to all the previous requests.
I've looked through the DF1 document and it says that the PLC 'has no data to transmit'.
Does anyone know exactly what this means?

Never mind, I see what it is.

Apparently, the PLC gets busy occasionally so you have to keep sending the 'Poll' (DLE ENQ stn BCC) until you get your data reply.