SAB
SAB
SAF
SAF
3.2 Half-Duplex SDLC-NRM Operation
82525
82526
82525
82526
The LAP controllers of the two serial channels can be configured to function in a half-duplex
Normal Response Mode (NRM), where they will operate as a slave (secondary) station, by
setting the NRM bit in the XBCH register of the respective channel.
In contrast to the full-duplex LAPB/LAPD operation, where the combined (primary +
secondary) station transmits both commands and responses and may transmit data at any
time, the NRM mode allows only responses to be transmitted
and
the secondary station may
transmit only when instructed to do so by the master (primary) station.
The HSCX gets the permission to transmit a frame from the primary by an S-, or
I-frame
with
the poll bit (p)
set!
The NRM mode can be profitably used in a point-to-multipoint configuration with a fixed
master-slave relationship and avoids collisions on the common transmit line. It’s the
responsibility of the master station to poll the slaves periodically and to process the error
recovery.
Prerequisite for NRM operation is:
auto-mode with 8-bit address field selected
MODE: MDS0, MDS1, ADM = 000
external timer mode
MODE: TDM = 0
same transmit and receive addresses, since only responses can be transmitted, i.e.
XAD1 = XAD2 = RAL1 = RAL2
←
(address
of secondary)
Note:
The broadcast address may be programmed in RAL2 if broadcasting is required.
Reception of Frames
The reception of frames functions equally to the LAPB/LAPD operation.
Transmission of Frames
The HSCX does
not
transmit S-, or
I-frames
if not instructed to do so by the primary station
sending an S-, or
I-frame
with the poll bit set.
The HSCX can be prepared to send an
I-frame
by the CPU issuing an XIF command (via
CMDR) at any time. The transmission of the frame, however, will not be initiated by the HSCX
prior to the reception of either a
RR, or
I-frame
with a poll bit set (p = 1).
After the frame has been transmitted (with the final bit set), the XFIFO is inhibited and the
HSCX waits for the arrival of a positive acknowledgement.
Semiconductor Group
34
901.90
SIEMENS [ Siemens Semiconductor Group ]
