KUNBUS-COM EtherCAT
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of all shift register modules receive the data from the input buffers in
the previous cycle. The input shift registers, on the other hand, utilise
the positive edges from the LOAD signal to copy all parallel input
values simultaneously to their output buffers. From there, they are
shifted bit by bit to the SSC input register area of the COMS-
Modulduring the current cycle.
The clock rates of the COMS-Moduls can be adapted manually or
automatically in 3 levels and are about 300, 1200 or 4800 Kbit/s. The
load impulse is between 5 and 15 µs long (active low). The delay
between the load edge (positive edge of the load impulse) and the
first clock edge (from high to low) is between 1 and 2 µs. These
values are completely uncritical when using the shift register modules
74HC165 (Input) and 74HC594 (Output).
An optional RESET line (Pin b7, active low) initialises the shift
register modules during the starting process of the COMS-Moduls
(i.e. also during each reset of the module).
With an arrangement of the output and input shift register as shown
in this example, all registers are switched in series so that the
COMS-Modulhas its own output data shifted back into the input
register again for checking purposes. A test sample shifted through
the complete chain without a LOAD signal allows the COMS-Modulto
detect how long the entire chain is by means of the necessary clock
signals for such a shifting procedure. A centre pickoff between the
output and input modules allows the COMS-Modul to also detect the
corresponding number of inputs and outputs during this run of a test
sample. If bit errors occur, the clock rate is reduced in automatic
mode by one level. Hence, with such a structure the COMS-
Modulcan find the right setting for the chain lengths and maximum
possible transmission rate independently. It is also possible,
however, to assign the lengths and clock rates manually via the CDI
menu. In this case, the centre pickoff can also be omitted (it is only
needed for determining the allocation between outputs and inputs).
The entire chain length is monitored constantly during ongoing
operation and must match the configured length. If the module
detects a difference, then it shuts down the SSC communication and
reports an error status via its status register.
The COMS-Modulcan operate a maximum of 32 input shift registers
plus 32 output shift registers. These limits must be observed.
Note on cycle time: The cycle time of the shift register interface is
normally independent of its chain length since the COMS-Modulin its
work cycle only starts the transmission of a shift procedure. The shift
Components
