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  It is important that the sender of the slave frame has the same conception of the data size as the bus master. Otherwise, it is not possible to send the slave frame. This results in a time out for the associated receiving DSPs and, eventually, the clearing of the VALID attribute (see Defining the Communication Interface for PM810 on page 101). A similar problem exists on the receiving size. Size mismatches can be avoided by assuring that associated receiving DSPs and “send” have exactly the same DAT elements connected (see Defining the Communication Interface for PM810 on page 101).

Message Transfer

 Message transfer is sent in an event driven manner, that is only when one or more communication interfaces have something to send. The contention problems that may arise when more than one communication interface has something to send are described in Event Location on page 288. As no slave frame can be larger than 32 bytes, messages must be divided into smaller parts, denoted packets, and assembled by the receiver. This is a higher layer protocol function, the description of which is beyond the scope of this document. Here, we shall be concerned with the transmission of packets only. When the bus master has established that a communication interface wants to send a packet, it sends a master frame with function code 12, and the address of a communication interface in question in the address/control field. 

The first two bytes of the slave frame contain the transmission mode (4 bits) and the target address (12 bits) of the frame. Several transmission modes are defined, however, the communication interface employs only the target addressing mode. This means that the frame is received by the communication interface the target address designates. This address is the very same address configured on the thumbwheel on the Advant Controller 110 stations (see Selecting Station Address on page 192). The communication interface with the designated address responds with a 32 byte slave frame containing 2 bytes of control information and 30 bytes of user data (see below).