Emerson SI-Universal User Manual page 80

P1 Device Type (1x.038) : EnDat, SSI, BISS*
There is a delay when the position information is obtained via a communications interface from an
encoder. It is assumed that the position information is taken from all types of encoder at a fixed
datum point during each sample period. The drive initiates the comms transfer at a suitable point in
advance of the datum to ensure that the position information is available when required.Correction
is then applied to the position information based on the change of position over the previous
sample and the advance time so that the position appears to have been sampled at the datum. If
the communications exchange, including allowing the encoder a recovery time (P1 Recovery Time
(1x.061)), is completed in 60 µs and the time required to obtain the full position is completed in 40
µs, then the position is sampled at each current controller task and P1 Low Speed Update Rate
Active (1x.063) = 0. Otherwise if the communication exchange is completed in 230 µs the position
is sampled every 250 µs and P1 Low Speed Update Rate Active (1x.063) = 1.
If the complete exchange takes any longer a trip is initiated. The following table shows the
calculations used by the drive to determine the necessary time to obtain the required data.
Protocol
t + t
ST D
where tcal ≤ t
Endat 2.1
encoder t + t
D cal
where t
t + t
ST D
where tcal ≤ t
EnDat 2.2
encoder t + t
D cal
where t
2T + t
BISS*
cal
t + T + NtT (t
D
SSI
value of 1.25μs is used)
* BiSS is not currently supported.
Where
Value
t
EnDat start time
ST
Transmission delay from the drive to
t
D
the encoder and back
T 1 / baud rate
t
Position calculation time
cal
Ns Single turn bits for a rotary encoder
Total number of position information
Nt
bits
tm Encoder recovery time
t
Time for additional information
Add
80
Time for full position
+ 10T + 2T + NtT + 5T
+ t /2+ 10T
ST D
+ 2T + NtT + 5T
> t + t /2 + 10T
cal ST D
+ 10T + 3T + NtT + 5T
+ t /2+ 10T
ST D
+ 3T + NtT + 5T
> t + t /2 + 10T
cal ST D
+ t + NtT + 10T
D
cannot be measured, and so a
D
Description
Time for complete data exchange
Time for full position + t
Time for full position + t
Time for full position + t
Time for full position + t
For 100 k baud = 5 us, 200 k baud = 2.5 μs, for
all other baud rates = 2 μs
P1 Line Delay Time (1x.062)
P1 Comms Baud Rate (1x.037)
P1 Calculation Time (1x.060)
P1 Comms Bits (1x.035)
−
P1 Rotary Turns Bits (1x.033)
P1 Comms Bits (1x.035)
P1 Recovery Time (1x.061)
t = 31T + t + 30T, t
Add1 ST
t depends on the value of P1 User Comms
Add
Enable (1x.067)
0/1: t = t
Add Add1
2: t = t + t
Add Add1 Add2
SI-Universal Encoder User Guide
m
+ t
Add
m
m
Source
= 31T
Add2
Issue: 2
m