Selecting An Absolute Encoder - Siemens SIMATIC FM 351 Installation Manual

10.4 Selecting an Absolute Encoder

Absolute Encoders
You can connect single-turn encoders (13 or 25 bit frame length) or multi-
turn encoders (25 bit frame length) to the FM 351 Positioning Function Mod-
ule (see Chapter 10.3).
Response Times
The following response times apply to absolute encoders (SSI):
Min. response time = Frame transfer time + Hardware switching time
Max. response time = 2 Frame transfer time + monostable flipflop period + Hardware switching time
With programmable absolute encoders:
Max. response period = Frame transfer time + Monostable flipflop period + Hardware switching time + 1/max. step
sequence frequency
Frame Transfer The frame transfer times depend on the baud rate:
Times
0.125 MHz
0.250 MHz
0.500 MHz
1.000 MHz
Example of The following example shows how you can calculate the minimum and maxi-
Response Times
mum response times. In the example a programmable encoder is not used.
Min. response time = 26 µs + 150 µs = 176 µs
Max. response time = 2 26 µs + 64 µs + 150 µs = 266 µs
Note
You can compensate the minimum response time by appropriate parameter-
ization of the switchover and switch-off differences.
FM 351 Positioning Function Module
EWA 4NEB 720 6001-02
Baud Rate Frame Transfer Time for
112 s
56 s
28 s
14 s
Hardware switching time: approx. 150 µs
Frame transfer time: 26 µs at 1MHz baud rate
Monostable flipflop period: 64 µs
Frame Transfer Time for
13 Bit 25 Bit
208 s
104 s
52 s
26 s
Encoders
10
10-5