Using the Data-Chaining Protocol

Data chaining is an optional protocol for transmitting a group of related messages. To send chained messages from the SLU, the SLU sets to 1 the begin-chain (BC) indicator for the message to indicate the first message in a chain. For all messages between the first and the last in the chain, the SLU sets the BC and the end-chain (EC) indicators to 0. For the last message in the chain, the EC is set to 1 again. When the SLU receives messages, it tests the chaining indicator to determine if the messages are chained.

The data-chaining protocol comprises three types of chains, as follows:
  • No-response chain. Each request in the chain is marked no response.
  • Exception-response chain. Each request in the chain is marked exception response.
  • Definite-response chain. The last request in the chain is marked definite response; all other requests in the chain are marked exception response.

When sending a message chain to the PLU, the SLU can send a CANCEL command if the SLU or the PLU finds a message error. If the SLU sends a CANCEL command to the PLU, the PLU discards all messages in the chain that it has received. If the PLU sends a negative response to any element of a chain, the SLU ends the chain normally or sends a CANCEL command.