Entry type: FAQ, Entry ID: 15364459, Entry date: 12/30/2016

What restrictions are there for active jobs when communicating with "WR_REC/"RD_REC" and "RDREC"/"WRREC" via PROFIBUS DP and PROFINET IO?

  • Entry
  • Associated product(s)
With "RDREC" (SFB52) / "RD_REC" (SFC59) you read data records from a central or remote slotted module. With "WRREC" (SFB53) / "WR_REC" (SFC58) you transfer data records to a central or remote slotted module.

"RDREC" (SFB52) / "RD_REC" (SFC59) and "WRREC" (SFB53) / "WR_REC" (SFC58) work asynchronously. The execution of an asynchronous instruction can extend over multiple calls. The CPU processes asynchronous instructions in parallel to the cyclic user program.

The number of active jobs for "RDREC" (SFB52) / "RD_REC" (SFC59) and "WRREC" (SFB53) / "WR_REC" (SFC58) depends on the CPU used.

The table below shows how many active jobs "RDREC" (SFB52) / "RD_REC" (SFC59) and "WRREC" (SFB53) / "WR_REC" (SFC58) your CPU supports at the same time.

System function (SFC)/
System function block (SFB ) /
"RDREC" (SFB 52) /
"WRREC" (SFB 53)

"RD_REC" (SFC 59)

"WR_REC" (SFC 58)

MeaningData record to/from central or remote slotted module (PROFIBUS DP or PROFINET IO)Data record to/from IO
IM 154 (ET 200pro)
IM 151 (ET 200S)
IM 147 (ET 200X)
4 jobs in total (RDREC, WRREC, RD_REC and WR_REC)

CPU 312, CPU 313,
CPU 314, CPU 315,
CPU 316

4 jobs in total (RDREC, WRREC, RD_REC, WR_REC)

CPU 317, CPU 319,
CPU 318-2

8 jobs in total (RDREC, WRREC, RD_REC, WR_REC)

CPU 41x1)

8 jobs each per PROFIBUS DP segment and PROFINET IO system

CPU 121x

4 jobs in total (RDREC, WRREC)

Not available

CPU 151x up to and including firmware V1.8

10 jobs each in total

10 jobs each in total

CPU 151x firmware V2.0 and higher

20 jobs each in total10 jobs each in total

Table 1

1) The number of simultaneous jobs on external PROFIBUS DP segments and PROFINET IO systems must not exceed 32 jobs per SFC/SFB. External PROFIBUS DP segments and external PROFINET IO systems are connected via the interfaces of CPs/CMs.

With a CPU 414-2DP, a maximum of 48 jobs per SFC/SFB can be executed at the same time (8 each on the two PROFIBUS DP segments that are connected to the integrated interfaces of the CPU, and 32 on external PROFIBUS DP segments and PROFINET IO systems).


  • The following holds for S7-400 CPUs:
    If the data source or data sink is in a central or expansion device (CR, ER), the CPU processes the jobs synchronously. In this case the jobs are processed sequentially and you do not have to take any restrictions on the number of jobs into account.

  • If you are operating multiple communication partners on the PROFIBUS network, then please make sure that never more than the specified jobs are active at the same time. Here one SFB, SFC or one instruction can run several CPU cycles long.
  • The restrictions listed in this entry for the active jobs "RDREC" (SFB52) / "RD_REC" (SFC 59) and "WRREC" (SFB53) / "WR_REC" (SFC58) also apply to the blocks that call these SFCs, SFBs and instructions internally. These include the blocks FM_CS, PID_FM and FMCS_PID.
    When communicating with an FM 355 (4 channels parameterized) via the FMCS_PID block, 4 read jobs are occupied.

The runtime of multiple RDREC / RD_REC or WRREC / WR_REC jobs running simultaneously can be longer than that of individual RDREC / RD_REC or WRREC / WR_REC jobs.