HEIDENHAIN PWM 20 - Manual (Page 231)

232 HEIDENHAIN ATS Software User's Manual
Continuous clock
For applications that require fast acquisition of the measured value, the EnDat interface can have
the clock run continuously. Immediately after the last CRC bit has been sent, the DATA line is
switched to HIGH for one clock cycle, and then to LOW. The new position value is saved with
the very next falling edge of the clock and is output in synchronism with the clock signal
immediately after the start bit and alarm bit. Because the MODE command "Encoder transmit
position value" is needed only once before the first data transmission, the continuous-clock
transfer mode reduces the length of the clock-pulse group by 10 periods per position value.
Synchronization of the serially transmitted code value with the incremental signal
Absolute encoders with EnDat interface can exactly synchronize serially transmitted absolute
position values with incremental values. With the first falling edge (latch signal
1)
) of the CLOCK
signal from the subsequent electronics, the scanning signals of the individual tracks in the
encoder and counter
2)
are frozen, as are the A/D converters for subdividing the sinusoidal
incremental signals in the subsequent electronics.
The code value transmitted over the serial interface unambiguously identifies one incremental
signal period. The position value is absolute within one sinusoidal period of the incremental
signal. The subdivided incremental signal can therefore be appended in the subsequent
electronics to the serially transmitted code value.
After power on and initial transmission of position values, two redundant position values are
available in the subsequent electronics. Since encoders with EnDat interface guarantee a
precise synchronization—regardless of cable length—of the serially transmitted absolute value
with the incremental signals, the two values can be compared in the subsequent electronics.
This monitoring is possible even at high shaft speeds thanks to the EnDat interface’s short
transmission times of less than 50 µs. This capability is a prerequisite for modern machine
design and safety systems.
Save new
position value
Save new
position value
CRC CRC Position value
n = 0 bis 7; depending on system Continuous clock