The Transaction Server message control system (MCS) is a Unisys product which, among other features, provides multiple logical connections between CANDE and a single station. Each logical connection is treated by CANDE as an individual station and is implemented as a data comm pseudostation. Transaction Server calls each connection a dialog of the CANDE window. Refer to the Transaction Server Operations Guide for information on logging onto Transaction Server, using windows and dialogs, and Transaction Server commands.

Pseudostations are also used for stations transferred from a foreign host across a BNA network. Because all input to and output from such stations is filtered through another MCS (for example, Transaction Server), there can be differences in behavior. In particular, commands intended for CANDE can instead be intercepted and acted upon by Transaction Server, and output for the station can be stored by Transaction Server, which can alter the usual pattern of flow control.

If any control commands do not yield normal results when entered from a Transaction Server window dialog, the problem can be resolved by using an extra control character (usually a question mark [?]). For example, Transaction Server intercepts a ?WRU command, but entering ??WRU directs the command to CANDE. Sometimes abbreviating the command can be helpful (Transaction Server does not recognize abbreviations). For example, a ?PURGE command will go to Transaction Server, but ?PURG will go to CANDE.

When a dialog of the CANDE window is opened, Transaction Server notifies CANDE of the usercode and if privileged or control status should apply. If the usercode does not require that a chargecode or accesscode be entered, CANDE then automatically logs on the user in a new session. Chargecodes and accesscodes are discussed later in this section. The CANDE HELLO command can be used to change to a different usercode.