10
10-5
FM 351 Positioning Function Module
EWA 4NEB 720 6001-02
10.4 Selecting an Absolute Encoder
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).
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
The frame transfer times depend on the baud rate:
Baud Rate Frame Transfer Time for
13 Bit
Frame Transfer Time for
25 Bit
0.125 MHz 112 s 208 s
0.250 MHz 56 s 104 s
0.500 MHz 28 s 52 s
1.000 MHz 14 s 26 s
The following example shows how you can calculate the minimum and maxi-
mum response times. In the example a programmable encoder is not used.
Hardware switching time: approx. 150 µs
Frame transfer time: 26 µs at 1MHz baud rate
Monostable flipflop period: 64 µs
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.
Absolute Encoders
Response Times
Frame Transfer
Times
Example of
Response Times
Encoders
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