Toggle Bit in AS-Interface Master/Scanner Command

[Ozpeter]

I have a printout of some Control Logix code that I am trying to implement in Emerson Rx3i. The code involves resetting the Master node of an Bihl+Wiedmann AS-i network via its command interface using the SET_OP_MODE and STORE_CDI commands

In the programming manuals I have seen, one aspect of sending a command is not very clear to me. This is the "T bit". The only description I have seen about this bit is:

Bit T in the command interface is the toggle bit. The toggle bit is only necessary in the case of interfaces which transfer data cyclically

Can anyone explain what this means?

The code I am dealing with issues an initial SET_OP_MODE with the T bit off, and then later issues a second SET_OP_MODE with the T bit on.

My gut says that this is simply an (how shall I say it?) .. oversight .. of the original author of the code I am looking at, and he second SET_OP_MODE command doesn't need the T bit set.