Updating the Process Image
Function
The K-Bus cycle (IP-Link) can be started cyclically (process image updating: free running ) or synchronously, with receipt of the DP-Data_Exchange telegram (process image updating: synchronous with cycles). If updating the process image is set to be synchronous with cycles, then the K-Bus cycle is started after receipt of the Data_Exchange telegram; otherwise the K-Bus cycle (IP-Link) is started independently of the Data_Exchange telegrams. The free running and synchronous procedures for updating of the process image are represented schematically below.
Free Run
Figure: Free-Run-Mode
Synchronous
Figure: The principle of updating the process image
Byte | Bit | Description |
|---|---|---|
9 | 6 | Updating of the process image |
Note Special attention must be paid to the running and reaction times of the whole system when parameterising the process image update function. |
Fast- Free- Run
In fast-free-run mode, the priority of the K-Bus/IP-Link is set higher. This activates the internal bus more frequently. This implies that the interfaces (such as KS2000 or DPV-1) are not dealt with so quickly. Fast-free-run mode is only permitted in asynchronous operation.
Figure: Fast-Free-Mode
|
Byte |
Bit |
Description |
|---|---|---|
|
9 |
4 |
Fast-Free-Run-Mode |
A certain running time is required to transmit the signals from the inputs to the controller and from the controller to the outputs. It is composed of a number of elements:
- Transmission from the controller to the Profibus master
- Transmission via Profibus (see Profibus reaction time) and
- Transmission from the Profibus box to the outputs (see K-Bus reaction time) / (see IP-Link cycle time)
The opposite sequence applies to the return journey.
Please see the data provided by the Profibus master manufacturer for the reaction time from the controller to the Profibus master. The times a comparatively short, and do not usually have to be taken into account.
Profibus reaction time
Profibus reaction time
The reaction time tDP on the Profibus is made up as follows. The constants A, B and TBYTE depend on the baud rate.
|
t = DP |
Constant A |
|
|
|
+ (constant B + (number of I/O bytes x TBYTE)) |
[Slave 1] |
|
|
+ (constant B + (number of I/O bytes x TBYTE)) |
[Slave 2] |
|
|
+ (constant B + (number of I/O bytes x TBYTE)) |
[Slave 3] |
|
|
+ (constant B + (number of I/O bytes x TBYTE)) |
[Slave n] |
|
Baud Rate |
Constant A (in ms) |
Constant B (in ms) |
TBYTE (in ms) |
|---|---|---|---|
|
9.6 kBaud |
64.5 |
25.6 |
1.15 |
|
19.2 kBaud |
32.3 |
12.8 |
0.573 |
|
93.75 kBaud |
6.6 |
2.62 |
0.118 |
|
187.5 kBaud |
3.3 |
1.31 |
0.059 |
|
500 kBaud |
1.6 |
0.49 |
0.022 |
|
1.5 MBaud |
0.67 |
0.164 |
0.00733 |
|
3 MBaud |
0.436 |
0.085 |
0.00367 |
|
6 MBaud |
0.27 |
0.044 |
0.00183 |
|
12 MBaud |
0.191 |
0.024 |
0.00092 |
K-Bus reaction time (K-Bus)
K-Bus reaction time (K-Bus)
The reaction time on the K-Bus (internal bus of the Compact Box) is determined by the shifting and saving of the data. The following table contains measured values for typical structures. It is possible to scale the calculation up for larger numbers.
Compact Box (numbers are for channels) | Running time on the K-bus | ||
|---|---|---|---|
Digital OUT | Digital IN | Analog IN/OUT | T_cycle (µs) |
4 | 0 | 0 | 150 |
8 | 0 | 0 | 170 |
12 | 0 | 0 | 170 |
16 | 0 | 0 | 200 |
20 | 0 | 0 | 200 |
24 | 0 | 0 | 220 |
28 | 0 | 0 | 220 |
32 | 0 | 0 | 245 |
0 | 4 | 0 | 150 |
0 | 8 | 0 | 180 |
0 | 12 | 0 | 180 |
0 | 16 | 0 | 200 |
0 | 20 | 0 | 200 |
0 | 24 | 0 | 230 |
0 | 28 | 0 | 230 |
0 | 32 | 0 | 250 |
4 | 4 | 0 | 170 |
8 | 8 | 0 | 195 |
12 | 12 | 0 | 220 |
16 | 16 | 0 | 250 |
20 | 20 | 0 | 275 |
24 | 24 | 0 | 300 |
28 | 28 | 0 | 325 |
32 | 32 | 0 | 350 |
4 | 4 | 1 | 630 |
4 | 4 | 2 | 700 |
Note For physical reasons, the K-Bus cycle is always at least about 90 µs. This means that the running or cycle time of the whole system, and in particular the cycle time for the PLC task, must be greater than the running time of the K-Bus cycle. If this is not the case, unstable conditions can arise in the input and output terminals of the PROFIBUS slave. |
IP-Link reaction time
IP-Link reaction time
The reaction time on the IP-Link is determined by the shifting and saving of the data. The following table contains measured values for typical structures. It is possible to scale the calculation up for larger numbers.
|
Extension Boxes added to Coupler Boxes |
Running time on the IP-Link | ||
|---|---|---|---|
|
Digital OUT (bits) |
Digital IN (bits) |
Analog IN/OUT (channels) |
T_cycle (µs) |
|
24 |
16 |
0 |
420 |
|
48 |
16 |
0 |
450 |
|
24 |
16 |
4 |
1500 |
|
48 |
16 |
16 |
2360 |
|
48 |
16 |
28 |
3500 |
|
48 |
16 |
40 |
4000 |
The settings for the Process image updating function are found in byte 9 of the User_Prm_Data (default settings are printed bold):
|
Bit no. |
Description |
is supported by |
|---|---|---|
|
Bit 6 |
Updating of the process image |
IP10xx, IP15xx; IP20xx, IP23xx, IP25xx, IP31xx, IP41xx, IP50xx, IP51xx, IP60xx |
In TwinCAT the settings for the PROFIBUS synchronisation DP/K-Bus cycle can be transferred with ADSRead and ADSWrite (ADS: Automation Device Specification). For this purpose they should be activated in the User_Prm_Data.
The settings for the Process updating function are found in the TwinCAT System Manager on the PrmData tab for the corresponding Busbox.
