12.5, 6.25, 3.125, and 1.5625 Hz (for a major frame comprising 32 minor cycles at 50 Hz). Whilst RTs cannot start a transfer directly on their own, the standard does include a method for when an RT needs to transmit data that is not automatically scheduled by the Bus Controller. These transfers are often called acyclic transfers as they are outside the structure used by the cyclic executive. In this sequence, an RT requests transmission through a bit in the status word, the Service Request bit. Generally, this causes the Bus Controller to transmit a Transmit Vector Word Mode Code command. However, where an RT only has one possible acyclic transfer, the Bus Controller can skip this part. The vector word is transmitted by the RT as a single 16-bit data word. The format of this vector word is not defined in the standard, so the system designers must specify what values from what RTs mean what action the Bus Controller is to take. This may be to schedule an acyclic transfer either immediately or at the end of the current minor cycle. This means that the Bus Controller has to poll all the Remote Terminals connected to the data bus, generally at least once in a major cycle. RTs with higher-priority functions (for example, those operating the aircraft control surfaces) are polled more frequently. Lower-priority functions are polled less frequently. Six types of transactions are allowed between the BC and a specific RT or between the Bus Controller and a pair of RTs: Controller to RT Transfer. The Bus Controller sends one 16-bit receive command word, immediately followed by 1 to 32 16-bit data words. The selected Remote Terminal then sends a single 16-bit Status word. RT to Controller Transfer. The Bus Controller sends one transmit command word to a Remote Terminal. The Remote Terminal then sends a single Status word, immediately followed by 1 to 32 words. RT to RT Transfers. The Bus Controller sends out one receive command word immediately followed by one transmit command word. The transmitting Remote Terminal sends a Status word immediately followed by 1 to 32 data words. The receiving Terminal then sends its Status word. Mode Command Without Data Word. The Bus Controller sends one command word with a Sub-address of 0 or 31 signifying a Mode Code type command. The Remote Terminal responds with a Status word. Mode Command With Data