Exchange/SNA Manual

Appendix D Type 1 Function Management
Headers
104700 Tandem Computers Incorporated D–1
This appendix describes the SNA type 1 function management (FM) header. The type
1 FM header is a six-byte control field that begins and ends all data streams from
Exchange/SNA to the host system other than console data. Console data is sent to the
host system without type 1 FM headers.
All data streams from the host system should begin and end with type 1 FM headers.
Any data received from the host that is not framed by type 1 FM headers is assumed to
be console data.
Table D-1 describes the fields that make up a type 1 FM header.
Table D-1. Type 1 FM Header
Byte Bits Meaning
0 .<0:7> %H06 Length of header
1 .<0> %B0 Not concatenated
1 .<1:7> %B0000001 FM header type
2 .<0:3> Medium or subdevice class
%B0000 Console
%B0001 Exchange (not supported by Exchange/SNA)
%B0010 Card (reader or punch)
%B0011 Printer
2 .<4:7> Subaddress (subdevice unit number)
3 .<0> SRI (not used by Exchange/SNA)
3 .<1> %B0 Demand Select (not supported)
3 .<2:7> Reserved
4 .<0:2> Destination Selection:
%B000 Resume (RDS)
%B001 End (EDS)
%B010 Begin (BDS)
%B011 Begin and End (only - ODS)
%B100 Suspend (SDS)
%B101 Abort (ABS)
%B110 Continue (CDS) (not supported)
%B111 Reserved
4 .<3> DST (not used by Exchange/SNA)
4 .<4> Reserved
4 .<5> CMI %B1 = compressed data
4 .<6> CPI %B1 = compacted data
4 .<7> Reserved
5 .<0:7> ERCL, Data record length, 0-255 (0 defaults to 80)