The SDLC loop mode of operation is described in the IBM document “Synchronous
Data Link Control General Information” GA27-3093. A subset
of this standard has been implemented. Only the primary station is
supported in loop mode. The “turn-around” signal described
in the SDLC document is not used.
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 | NOTE: The SDLC loop mode is not available for use on the 2-Channel
ACC interface. |
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Frame Types Supported |
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In loop mode the HDLC/NRM protocol supports the use of the
following additional commands and responses:
Commands | Responses |
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SIM | RIM |
TEST | TEST |
UP | |
Request to Send Control |
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In Primary/Loop mode the HDLC/NRM protocol maintains the RTS signal
in the ON condition.
Unnumbered Poll |
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In Primary/Loop mode the HDLC/NRM protocol will poll the secondary stations
with the Unnumbered Poll (UP) command with the P bit set to '0' and
with the address field set to 0xFF.
TEST Command and Response |
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In Primary/Loop mode the HDLCNRM protocol will periodically
issue a TEST command with the P bit set to '0' to
each secondary station. If the correct response (TEST response with
the F bit set to '0') is not received within the
T1 timeout period, the TEST command will be repeated. After N2 repeat
transmissions of the TEST command with no response received the
secondary station will be considered to be in the disconnected mode.
The default value for the contents of the TEST command I-field
is six ASCII characters as follows:
“U TEST”
The test pattern can be modified to any sequence of up to
6 bytes by the use of a control write request to the terminal. The
format of the control write request is as follows:
This request should be the first request to the terminal after
it is enabled.
TEST commands are transmitted to secondary stations in the “up” state (i.e.
after a successful SNRM/UA exchange) at 60 second intervals.The Information
field of the TEST response is not verified.
SDLC Frame Types Not Supported |
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The following commands and responses defined in the IBM SDLC specification
are not supported:
Commands | Responses |
|---|
CFGR | BCN |
XID | CFGR |
REJ | RD |
| XID |
| REJ |
Modulo-128 sequence numbering is also not supported.
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