HEIDENHAIN PWM 20 - Manual (Page 234)

November 2014 Interface description 235
7.4.2 Synchronous serial interface; programming via connector
The absolute position value beginning with the Most Significant Bit (MSB first) is transferred
on the DATA lines in synchronism with a CLOCK signal transmitted by the control. The SSI
standard data word length for singleturn encoders is 13 bits, and for multiturn encoders 25 bits.
In addition to the absolute position values, incremental signals can be transmitted.
See Incremental signals for a description of the signals.
For the ECN/EQN 4xx and ROC/ROQ 4xx rotary encoders, the following functions can be
activated via the programming inputs of the interfaces by applying the supply voltage Up:
 Direction of rotation
Continuous application of a HIGH level to pin 2 reverses the direction of rotation for ascending
position values.
 Zeroing (datum setting)
Applying a positive edge (t
min
> 1 ms) to pin 5 sets the current position to zero.
Note:
The programming inputs must always be terminated with a resistor (see input circuitry of the
subsequent electronics).
Control cycle for complete data format
In the quiescent state clock and data lines are at high-level. The internally and cyclically formed
position value is stored on the first falling edge of the clock. The stored data is then clocked out
on the first rising edge.
After transmission of a complete data word, the data line remains low for a period of time (t
2
)
until the encoder is ready for interrogation of a new value. Encoders with the SSI 39r1 or SSI
41r1 interface additionally require a subsequent clock pause t
R
. If another data-output request
Interface SSI serial
Ordering designation Singleturn: SSI 39r1
Multiturn: SSI 41r1
Data transfer Absolute position values
Data input Differential line receiver according to EIA standard RS 485 for
the CLOCK and CLOCK signals
Data output Differential line driver according to EIA standard RS485 for
DATA and DATA signals
Code Gray code
Ascending position values With clockwise rotation viewed from flange side
(can be switched via interface)
Incremental signals Device-dependent
1 Vpp, TTL, HTL (see the respective Incremental signals)
Programming inputs
Inactive
Active
Switching time Direction of rotation and zero reset (for ECN/EQN 4xx, ROC/
ROQ 4xx)
LOW < 0.25 x Up
HIGH > 0.6 x Up
t
min
> 1 ms
Connecting cable
Cable length
Propagation time Shielded HEIDENHAIN cable
e.g. PUR [(4 x 0.14 mm
2
) + 4 (2 x 0.14 mm
2
) + (4 x 0.5 mm
2
)]
max. 100 m with 90 pF/m distributed capacitance
6 ns/m