Rockwell Automation 20-COMM-S RS-485 DF1 Adapter - Manual (Page 56)

Glossary-6
T Type 0/Type 1/Type 2 Control
When transmitting I/O, the adapter can use different types of messages
for control. The Type 0, Type 1, Type 2 control events help
Allen-Bradley personnel identify the type of messages that an adapter is
using.
Transaction Number (TNS)
A 2-byte field in a DF1 message packet that is used in combination with
the SRC (source node of the message) and CMD (command code) bytes
to uniquely identify every message packet. The TNS is typically
incremented each time a new DF1 message is sent. The reply message
will contain the same TNS value.
Z
Zero Data
When communications are disrupted (for example, a cable is
disconnected), the adapter and drive can respond with zero data. Zero
data results in the drive receiving zero as values for command data. If the
drive was running and using the Reference from the adapter, it will stay
running, but at zero Reference.