SAB
SAB
SAF
SAF
2
Operating Modes
82525
82526
82525
82526
The HDLC controller of each channel can be programmed to operate in various modes, which
are different in the treatment of the HDLC frame in receive direction. Thus, the receive data
flow and the address recognition features can be effected in a very flexible way, which satisfies
most requirements.
There are 6 different operating modes which can be set via the MODE register.
2.1 Auto-Mode (MODE: MDS1, MDS0 = 00)
Characteristics: Window size 1, arbitrary message length, address recognition.
The HSCX processes autonomously all numbered frames (S-,
I-frames)
of an HDLC
procedure.
The HDLC control field, data in the I-field of the frames and an additional status byte is
temporarily stored in the RFIFO. The HDLC control field as well as additional information can
also be read from special registers (RHCR, RSTA).
According to the selected address mode, the HSCX can perform a 2-byte or 1-byte address
recognition. If a 2-byte address field is selected, the high address byte is compared with the
fixed value FE
H
or FC
H
(group address) as well as with two individually programmable values
in RAH1 and RAH2 registers. According to the ISDN LAPD protocol, bit 1 of the high byte
address will be interpreted as COMMAND/RESPONSE bit (C/R), dependent on the setting of
the CRI bit in RAH1, and will be excluded from the address comparison.
Similary, two compare values can be programmed in special registers (RAL1, RAL2) for the
low address byte. A valid address will be recognized in case the high and low byte of the
address field correspond to one of the compare values. Thus, the HSCX can be called
(addressed) with 6 different address combinations, however, only the logical connection
identified through the address combination RAH1, RAL1 will be processed in the auto-mode,
all others in the non-auto mode. HDLC frames with address fields that do not match with any
of the address combinations, are ignored by the HSCX.
In case of a 1-byte address, RAL1 and RAL2 will be used as compare registers. According to
the X.25 LAPB protocol, the value in RAL1 will be interpreted as COMMAND and the value in
RAL2 as RESPONSE.
After receiving a frame it takes 5 clock cycles to generate the response frame and to start
transmission.
2.2 Non-Auto Mode (MODE: MDS1, MDS0 = 01)
Characteristics: address recognition, arbitrary window size.
All frames with valid addresses (address recognition identical to auto-mode) are forwarded
directly to the system memory.
The HDLC control field, data in the I-field and an additional status byte are temporarily stored
in the RFIFO. The HDLC control field and additional information can also be read from special
registers (RHCR, RSTA).
In non-auto mode, all frames are treated similarly.
Semiconductor Group
24
SIEMENS [ Siemens Semiconductor Group ]
