Host SNA Session Considerations

Outbound Requests

Outbound FMD requests which carry FIC/OIC are expected to begin with a 3270 write-type command and a WCC. The COPY and WSF commands are not supported, and are rejected with sense code X'1003.

Only one HSM command can be sent outbound per chain; chain elements are concatenated in the HSM and the data is not given to the command processor until the LIC element is received.

Inbound Requests

Inbound reply requests which carry FIC/OIC always begin with the following sequence:

                                       EBDIC                                   ASCII

AID: (‘Enter’)                             X'7D                                     X'27

Cursor address:                        X'40C1                                 X'2041

SBA order:                                X'1140C1                             X'112041

The HSM, unlike a true 3274, is strictly "speak-when-spoken-to".  Therefore, when an FMD request that contains a valid HSM command is sent outbound, the HSM expects to return a reply FMD request inbound.  This means that the Host request must leave the HSM in either CONTENTION or SEND state (by carrying either EB or CD).

If the HSM cannot return its reply because the Host left it in RCV state, it sends the DFC command ‘Signal' (X'00010000) to solicit a change of direction.  The appropriate Host action is to send a null-RU or Write (with no data) request carrying EB or CD; the HSM then sends its pending reply. The HSM cannot buffer more than one reply, so if the Host request following an HSM-originated ‘Signal' contains an HSM command, the latter is discarded.

SNA Command Sequences

_{Figure 4 - SNA Command Sequences}

Note that no BIND was sent outbound in the example in Figure 4 - SNA Command Sequences. The HSM is, by convention, the contention winner. However, because it never sends unsolicited requests inbound, the Host can always begin a bracket if in the BETB state.