Use direct Load/Transfer instructions for a two-word PPO3 process-data exchange. Use SFC14/SFC15 only when the configured PROFIBUS area must be transferred consistently and its length matches the functions’ supported transfer sizes. For the MICROMASTER 420 arrangement described here, PPO1 separates an eight-byte PKW area from a two-word PZD area; selecting PPO1 does not make the entire telegram one undivided SFC transfer.
PPO3: Keep the control and setpoint words separate
The described PPO3 process-data area contains two words: the control word at PQW20 and the speed reference at PQW22. Continue writing these values with Load/Transfer instructions, called Move instructions in LAD. The cited SFC14/SFC15 length restriction excludes word- and double-word-sized transfers: the supported lengths are three bytes or at least five bytes.
PQW20 := control_word
PQW22 := speed_referenceSelect the required control-word value in PLC logic, such as 16#47E for Ready, 16#47F for forward, or 16#C7F for backward. These are values written to the control word; they are not PROFIBUS addresses.
PPO1: Treat PKW and PZD as separate zones
PPO1 divides communication into a four-word PKW parameter area and a two-word PZD process-data area. The PKW area is consistent over its total eight-byte length and transfers one parameter transaction at a time. Build that complete four-word structure in a data block, then transfer it as one consistent message with SFC14/SFC15.
| Zone | Length | Purpose | Supported handling |
|---|---|---|---|
| PKW | Four words / eight bytes | Parameter access | Build the complete structure in a DB and transfer it consistently with SFC14/SFC15 |
| PZD | Two words | Control word and speed reference | Use Load/Transfer for the described two-word exchange |
The evidence does not define the individual PKW fields or the SFC interface parameters. Obtain those definitions from the applicable Siemens documentation before constructing the DB; do not infer the telegram layout from the PZD addresses.
Encode the speed reference as a normalized word
The speed reference represents a fraction of the maximum frequency configured in the drive. The supported mappings in the evidence are:
| Reference word | Commanded frequency | If maximum frequency is 50 Hz |
|---|---|---|
W#16#0000 |
Standstill | 0 Hz |
W#16#1000 |
25% of maximum | 12.5 Hz |
W#16#2000 |
50% of maximum | 25 Hz |
W#16#4000 |
100% of maximum | 50 Hz |
The stated selectable range is W#16#0000 through W#16#5000. Under PPO3, write the selected word directly to PQW22. Do not convert the decimal address 22 into the hexadecimal value 16#16; the address identifies the output word, while the hexadecimal literal is the data carried by that word.
Selection and verification procedure
- Confirm whether the configured telegram is PPO3 or PPO1.
- For PPO3, write the control word to
PQW20and the normalized speed reference toPQW22with Load/Transfer instructions. - For PPO1 parameter access, create the complete four-word PKW structure in a DB and transfer all eight bytes consistently with SFC14/SFC15.
- Verify that control direction changes when the selected control word changes and that the drive frequency follows the speed-reference percentage of its configured maximum frequency.
FAQ
Can SFC14 and SFC15 transfer the two PPO3 words?
Not in the described configuration. The two-word PZD area is double-word sized, while the cited SFC restriction supports three-byte transfers or transfers of at least five bytes; use Load/Transfer instructions.
How do I use SFC14 and SFC15 with PPO1?
Build the complete four-word, eight-byte PKW telegram in a DB and transfer it as one consistent message. Keep the separate two-word PZD control and speed area on its established handling path.
What speed-reference word commands 50 percent speed?
Write W#16#2000 for 50% of the drive’s configured maximum frequency. If that maximum is 50 Hz, the resulting reference is 25 Hz.