SIGNAL protocol

IMS sends the SIGNAL command to request change-direction (SIGNAL RCD - X'00010000') at the end of an input message chain that does not indicate change-direction and that produces output that must be sent prior to subsequent input.

For example, IMS replies to input commands, response mode transactions, conversational transactions, or input while in test mode. SIGNAL RCD is sent before IMS sends any required DR1 or DR2 response for the input message.

Based on the IMS response protocol and the point at which IMS sends SIGNAL RCD, IMS requires that the next input following SIGNAL RCD be either LUSTATUS or CHASE indicating CD or EB. If neither of these is received by IMS, the session is terminated. Change-direction allows IMS to send the pending output message; end-bracket causes the message to be dequeued and the conversation mode, response mode, or test mode to be terminated. No other input can be processed until the pending output message has been successfully transmitted or dequeued.

After receiving SIGNAL RCD while in bracket state, IMS sends change-direction either at the end of the current output message or immediately on receipt of an LUSTATUS. A SIGNAL RCD received while between brackets prevents IMS from sending further output messages until normal flow input (data or SNA command) occurs.