SAB 82525
SAB 82526
SAF 82525
SAF 82526
3
Procedural Support (Layer-2 Functions)
When operating in the auto-mode, the HSCX offers a high degree of procedural support. In
addition to address recognition, the HSCX autonomously processes all (numbered) S- and
I-frames (prerequisite window size 1) with either normal or extended control field format
(modulo 8 or modulo 128 sequence numbers – selectable via RAH2 register).
The following functions will be performed:
– updating of transmit and receive counter
– evaluation of transmit and receive counter
– processing of S commands
– flow control with RR/RNR
– generation of responses
– recognition of protocol errors
– transmitting of S commands, if acknowledgement is missing
– continuous status query of opposite termination after RNR has been received
– programmable timer/repeater functions.
In addition, all unnumbered frames are forwarded directly to the processor.
Additional logic connections can be operated in parallel by software. The logic link can be
initialized by software at any time (RHR).
3.1 Full-Duplex LAPB/LAPD Operation
Initially (i.e. after RESET), the LAP controllers of the two serial channels are configured to
function as a combined station, where they autonomously perform a subset of the X.25 LAPB/
ISDN LAPD protocol.
Reception of Frames
The logic processing of received S-frames is performed by the HSCX without interrupting the
µC. The µC is merely informed by interrupt with respect to status changes in the opposite
station (receive ready/not receive ready) and protocol errors (unacceptable N(R) or S-frame
with I field).
I-frames are also processed autonomously and checked for protocol errors. The I-frame will
not be accepted in the case of N(s) error (no interrupt is forwarded to the µC), but is
immediately confirmed by an S response. If the µC sets the HSCX into a "receive not ready"
status, an I-frame will not be accepted (no interrupt) and an RNR response is transmitted.
U-frames are always stored in the RFIFO and forwarded directly to the µC. The logic sequence
and the reception of a frame in the auto-mode is illustrated in figure 10.
Note: The state variables N(S), N(R) are evaluated within the window size, i.e. the HSCX
checks only the Isb of the receive and transmit counter regardless of the selected
modulo count.
Semiconductor Group
28
SIEMENS [ Siemens Semiconductor Group ]
