88TK50/R1210
2
PROCONTROL remote---bus coupling
Sending from the station
For send data communication, the station protocol control
offers two modes:
•
Cyclic mode
•
Event mode.
In cyclic mode, the station protocol control allocates the send
permit (activated by cyclic calls) to the station modules, one
after the other, allowing them to send their cyclic telegrams
which are then stored in the send shared memory by the
station---bus interface.
At intervals of approx. 30 msec, the FDDI system interface
sends the cyclic telegrams available in the send shared
memory, packed in FDDI frames, with broadcast addressing to
the FDDI ring via the standard interface.
The number of telegrams to be transmitted per time interval is
calculated by the 88TK50 based on the system parameters
and the number of cyclic send telegrams from the station.
In event mode, the station protocol control (activated by event
calls) grants a send permit to those modules, which have indi-
cated an event, allowing them to send their event telegrams.
These telegrams will be stored by the station---bus interface in
the event---send FIFO. Entries in the event---send FIFO simul-
taneously update the entries in the cyclic send shared
memory.
Within the token rotation time of the FDDI token, the FDDI sys-
tem interface then sends the event telegrams available in the
event---send FIFO, packed in FDDI frames with broadcast
addressing, to the FDDI ring via the standard interface.
During this procedure, all telegrams are checked for accuracy
as to formal criteria and to their contents after they have been
transferred over the station bus.
In the case of a fault, the telegram is inhibited through the
station---bus protocol control and, consequently, not sent onto
the FDDI ring.
Sending to the station
The module receives all PROCONTROL telegrams, packed in
FDDI frames which are transferred over the FDDI ring.
The FDDI data unit of a received frame determines, whether
and in which receive shared memory the telegrams packed in
the frame shall be made available to the station---bus interface
for sending.
FDDI frames with packed event telegrams will be given priority.
When the event data are being distributed, the cyclic PRO-
CONTROL receive shared memory for cyclic data is updated
immediately so that no previous values will be transmitted
when distribution from the cyclic receive shared memory is
resumed.
A distinction is being made between FDDI frames with broad-
cast addressing and ones with destination addressing.
Telegrams with process data are transmitted by means of
FDDI frames with broadcast addressing. Telegrams with list
data are transmitted by means of FDDI frames with destination
addressing.
This means that list data are received only in stations with the
respective destination address.
The FDDI system interface unpacks the telegrams and makes
the cyclic telegrams in the cyclic receive shared memory, the
event telegrams in the event---receive FIFO and the list tele-
grams in the list---receive shared memory available to the
station---bus interface for sending.
The cyclic telegrams in the cyclic receive shared memory are
made available station---by---station one after the other which
means that the cyclic telegrams will also be sent station---by---
station over the station bus.
The formal accuracy of the received FDDI frames is checked
based on a frame check sequence which is used to secure the
telegrams packed in the FDDI frames.
If any error is detected in an FDDI frame addressed to the
station, the telegrams of this frame will not be stored in the
receive shared memory. The FDDI system interface, however,
will initiate the following by setting the fault flag in the frame
status field:
--- In the case of a frame with event telegrams, the faulty
frames received are repeated,
--- In the case of a frame with cyclic telegrams, a new cyclic
frame with the current data of the station will be transmitted.
The station---bus interface sends the telegrams filed in the
receive shared memories over the station bus, the event
telegrams taking priority.
On that occasion, formal accuracy of the telegrams is checked
based on the check characters protecting the telegram
contents (operation code, addresses, data) by the receiving
modules.
If an error has been detected, these telegrams will not be eval-
uated.
The station cycle starts with a ’start cycle’ telegram and ends
with the ’start cycle’ telegram of the next one.
The ’start cycle’ telegram is generated by the 88TK50 after all
cyclic send telegrams and all receive telegrams have been
transferred once within the station.
Due to the interruptions caused by the transmission of event
telegrams and list telegrams, the length of a station cycle is
determined by the number of cyclic send telegrams of that
specific station and of the other stations as well as by the
number of event telegrams and list telegrams to be trans-
ferred.
