The order in which OpenTP1 sends messages to connected logical terminals is based on a combination of first-in first-out (FIFO) and message priority.

The segments in a message are sent continuously. After one message has been sent, the next message is sent to a logical terminal. You use the order operand of the -c option in the UAP common definition (mcfmuap) to specify the order in which transmission requests from multiple UAPs are to be processed. Figure 3-38 illustrates the sending order.

Figure 3-38 Order of sending MCF messages to logical terminals

The type of message to be sent by the UAP determines its priority. When different types of messages are waiting to be sent to the same logical terminal, the message with the highest priority is sent first. When the same type of messages are waiting, they are sent in FIFO order.

Table 3-7 lists the sending priority of the different types of messages.

Table 3-7 Message type and sending priority

Message type	Sending priority
Response message	High Medium Low
Send-only message (with priority)
Send-only message (normal) or inquiry message

Figure 3-39 illustrates the order in which OpenTP1 sends messages to another system. In the diagram, messages are written to the output queues in the order of A-B-C; however, the messages are sent to the logical terminals in the order of B-C-A. B and C are sent before A because they have higher sending priorities. B is sent before C based on the FIFO method: that is, B was written to the output queue before C.

Figure 3-39 Sending order based on sending priority combined with FIFO