Description
The maximum number of connections and communication jobs that an S7-300 or S7-400 supports depends on the CPU and CP used.
Each connection requires connection resources for the end point or for the transition point (e.g. CP) on the stations concerned. The number of connection resources is CPU/CP-specific. If all the connection resources of a communication partner are occupied, no new connection can be set up.

More information on the maximum number of connection resources is available in the Technical data of the S7-300 CPUs.

More information on the maximum number of connection resources is available in the Technical data of the S7-400 CPUs.

An overview of the services and quantity frameworks that the CPUs with integrated PN interface support is available in Entry ID: 18909487.

Information on which communication connections of the Industrial Ethernet CPs need connection resources of the CPU is available in Entry IDs: 42480411 and 42480718.

Information on the maximum permitted number of simultaneous connections via CP is available in the relevant CP manuals in the chapter entitled "Performance Data".

An overview of the services and quantity frameworks that the Industrial Ethernet CPs of S7-300 and S7-400 support is available in Entry IDs: 16767769 and 15368142.

Description
One connection resource is required in the S7-300 CPU for each of the following communication connections of the Industrial Ethernet CP.
 

Communication connection	Service	TSAP
PG communication via S7 server	Web diagnostics, for example for displaying the diagnostics buffer of the CPU Set time in CPU 318 Applets, for example to read MLFB or status of the CPU Bacnet (read system status list (SSL)) ERPC (read system status list (SSL))	0x01
System connection via S7 server	FTP server (read and write access to file DBs) FTP client (read and write access to file DBs) Applets (read and write access to CPU data) Bacnet (read and write access to CPU data) ERPC (read and write access to CPU data)	0x03
System connection	FETCH and WRITE via SEND/RECEIVE connection PBC for OP multiplexer	0x03

If the FETCH / WRITE, PBC for multiplexer, FTP and Web diagnostics services are used simultaneously in the Industrial Ethernet, 4 connection resources of the S7-300 CPU are occupied.
An Industrial Ethernet CP can occupy, for example, a maximum of 4 connection resources in the S7-300 CPU; this one connection resource each for:

• PG communication via S7 server (TSAP=0x01)
• System connection via S7 server (TSAP=0x03)
• System connection SEND/RECEIVE (TSAP=0x03)
• System connection OP multiplexer (TSAP=0x03)

Description:
This entry gives you an overview of the system limits of S7 Communication.

The following figures show the basic configuration of S7 Communication between F CPUs via Industrial Ethernet. Bidirectional data communication is via an S7 connection.

Fig. 01

Alternatively, bidirectional data communication can be via two separate S7 connections. In this way, you can structurally separate the send and receive channels, for example.

Fig. 02

The system limit of S7 Communication is determined by the following parameters:

• Max. number of connections supported by the CPU.
• Max. number of S7 connections that can be configured per interface.
• Max. number of instances supported by the CPU.

Max. number of connections supported by the CPU
The following table shows the maximum number of connections supported by the F CPUs.
 

F CPU	Max. number of connections
IM 151-8F CPU	12
CPU 315F-2 PN/DP	16
CPU 317F-2 PN/DP	32
CPU 319F-3 PN/DP	32
CPU 416F-2 DP	64
CPU 416F-3 PN/DP	64
WinAC RTX F 2009	64

Max. number of S7 connections that can be configured
The following table shows the maximum number of S7 connections supported by the F CPUs.
 

F CPU	Max. number of S7 connections that can be configured
IM 151-8F CPU	10
CPU 315F-2 PN/DP	14
CPU 317F-2 PN/DP	16
CPU 319F-3 PN/DP	16
CPU 416F-2 DP with CP443-1 Adv.	62
CPU 416F-3 PN/DP	30
WinAC RTX F 2009	Via CP5611: 6 Via CP5613: 48 Via CP1616: 30 Via IE general: 14

Max. number of instances
The following table shows the maximum number of instances supported by the F CPUs.
 

F CPU	Max. number of instances
IM 151-8F CPU	32
CPU 315F-2 PN/DP	32
CPU 317F-2 PN/DP	32
CPU 319F-3 PN/DP	32
CPU 416F-2 DP with CP443-1 Adv.	Firmware version < V5.2: 1800 (can be configured: 600 preset) Firmware version V5.2 onwards: 4000 (can be configured: 600 preset)
CPU 416F-3 PN/DP	600 (internal interface)
CPU 416F-3 PN/DP with CP443-1 Adv.	Firmware version < V5.2: 1800 (can be configured: 600 preset) Firmware version V5.2 onwards: 4000 (can be configured: 600 preset)
WinAC RTX F 2009	600 (can be configured: 300 preset)

Example:
In a CPU 319F-3 PN/DP, you select S7 Communication via TCP/IP for safe bidirectional data communication. Depending on whether the data communication is via one or two configured S7 connections, you can configure another 15 or 14 S7 connections.

The fail-safe communication blocks "F_SENDS7" and "F_RCVS7" are called in the CPU user program for safe bidirectional data communication via S7 connections. These blocks internally call the system function blocks SFB8 "USEND" and SFB9 "URCV" respectively. In this way, the user data and associated acknowledgments are sent and received. An instance data block is assigned to each SFB8 "USEND" and SFB9 "URCV" system function block. Thus, the number of instance data blocks (= instances) is identical to the number of communication jobs.

This means that in the case of safe bidirectional data communication, at least four communication jobs are executed and four instances are needed. In this case, with the CPU 319F-3 PN/DP there remain 28 free instances.

In the user program of the CPU 319F-3 PN/DP, you can call a maximum of 16 fail-safe communication blocks "F_SENDS7" or F_RCVS7", because the maximum of instances is limited to 32.
In the case of safe bidirectional data communication, the CPU 319F-3 PN/DP can communicate with a maximum of 8 F CPUs.

Calculation for safe bidirectional data communication in the CPU 319F-3 PN/DP:
8  "F_SENDS7" + 8 "F_RCVS7" = 16 fail-safe communication blocks
8*("USEND" + "URCV") + 8*("USEND" + "URCV")
= 16 "USEND" + 16 "URCV" = 32 communication jobs or instances

Note:
The safety function is foremost with F CPUs. Therefore, the system limit of S7 Communication is determined not only by the number of communication connections, but also by the response times achieved. If the required response times are not achieved because of the number of communication connections, the remedy is to

• Reduce the number of communication connections.
• Use a larger and faster CPU.

Description:
The example shown here explains how you can use your project to determine the size of the load memory and main memory in order to define the size of the memory cards needed, for example.

1. Open the project and mark the block folder.
    
2. Right-click and select "Object Properties".
   
   
   Fig. 01
    
3.  In the new window you select the "Blocks" tab.
   
   
   Fig. 02
    
4. You can also determine the size of the load memory by adding the user program and the system data (red marking),
   for example, 428 bytes + 710 bytes = 1138 bytes
   The memory card used in the S7 CPU must have this size at least.
    
5. The required size of the CPU's main memory is displayed directly (yellow marking).

Keywords:
Memory size, MMC, MC

Configuration Notes:
System functions and blocks SFB52 "RDREC" / SFC59 "RD_REC" (read record) are used to read data records of a component (module) of a DP slave/PROFINET IO device. System functions and blocks SFB53 "WRREC" / SFC58 "WR_REC" (write record) are used to write data records to a component (module) of a DP slave/PROFINET IO device.

Depending on the CPU used, the number of active jobs of the system functions and blocks SFB53/SFC58 and SFB52/SFC59 is limited.

The following table provides information about how many active jobs of the system functions and blocks SFB53/SFC58 and SFB52/SFC59 your CPU supports simultaneously.
 

System function/ system block	SFB 52 "RDREC"/ SFB 53 "WRREC"	SFC 59 "RD_REC"/ SFC 58 "WR_REC"
Explanation	Data record from DP slave, PROFINET IO device	Data record from DP slave
IM154 (ET 200pro) IM151 (ET 200S) IM147 (ET 200X)	4 jobs together with SFC 58/59	4 jobs together with SFB 52/53
CPU 312, CPU 313, CPU 314 CPU 315, CPU 316	4 jobs together with SFC 58/59	4 jobs together with SFB 52/53
CPU 317, CPU 319 CPU 318-2	8 jobs together with SFC 58/59	8 jobs together with SFB 52/53
CPU 41x1)	8 jobs each per PROFIBUS DP segment and PROFINET IO system	8 jobs each per PROFIBUS DP segment and PROFINET IO system

¹⁾ The number of simultaneous jobs on external PROFIBUS DP segments or PROFINET IO systems must not exceed 32 jobs per SFC/SFB.

Example:
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).

Rules:

• There are no restrictions for simultaneous jobs in the subracks (CR, ER). The SFCs run synchronously via the backplane bus. Any number of synchronous SFCs can be called.
• 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 SFC/SFB can run several CPU cycles long.
• The restrictions listed in this entry for the active jobs of the system functions and blocks SFB53/SFC58 and SFB52/SFC59 also apply to the blocks that call these system functions and blocks internally. These include the blocks FM_CS, PID_FM and FMCS_PID, for example.
  Example:
  When communicating with an FM 355 (4 channels parameterized) via the FMCS_PID block, 4 read jobs are occupied.

Note:
System functions SFC58/59 are available on all CPUs.

Description:
The transmission time on the PROFIBUS and Industrial Ethernet depends on the cycle times of the implemented blocks (S7-300 or S7-400 CPU, PROFIBUS CP, Industrial Ethernet CP) and the volume of data to be transferred.

The following entry gives you a tool for determining transmission time for typical configurations on PROFIBUS 22180794.

The following entry gives you a tool for determining transmission time for typical configurations on the Industrial Ethernet: 22180793.

Instructions:
Events can occur in the current process which require responses which are quicker than are possible in the current program cycle. Events also occur which do not last long enough to be identified in the current program cycle. Therefore, there is hardware interrupt processing in SIMATIC S7-300 controllers.
Together with:

• analog input modules (AI),
• digital input modules (DI) and
• function modules (FM)

with hardware interrupt capability, a program which is adapted to suit the event can be called in real time.
Hardware interrupts approximate to interrupts.
This entry is intended to serve as a guide to hardware interrupts in S7-300 CPUs.

General:
If an alarm-triggering event occurs during program processing, the operating system calls the alarm OB 40, interrupting the processing of the program cycle or lower-priority program blocks. The alarm-triggering event (or events (multiple bits can be set)) is/are specified more precisely via the alarm OB 40's temporary local data. The temporary local data can be evaluated by the user program in the alarm OB.
If there is no alarm OB 40 in the CPU when an alarm-triggering event occurs, the CPU goes into STOP mode. The alarm OB has a fixed priority setting of 16.

Hardware interrupt-triggering events in the different modules:

Analog input modules: A value can be monitored in analog input modules with hardware interrupt capability. The hardware interrupt can be configured to be triggered off if values drop below or rise above specified thresholds. More information about the individual analog input modules is available in the manual "Programmable Logic Controller S7-300 Module Data" in Entry ID 8859629 in chapter 4.

Digital input modules: Individual bits can be monitored in digital input modules with hardware interrupt capability. The hardware interrupt can be configured to be triggered off in the event of a negative or positive edge to the bit. More information about the individual digital input modules is available in the manual "Programmable Logic Controller S7-300 Module Data" in Entry ID 8859629 in chapter 3.

Function modules: Since function modules with hardware interrupt capability perform a wide range of different tasks, allowing the hardware interrupts to be configured for different events, it is advisable to consider the FM 350-1 counter module by way of an example.
The FM 350-1 enables a hardware interrupt to be triggered off in the CPU whenever comparison values are reached, or in the event of an over-run or under-run and/or if the counter passes through zero. More information about the FM 350-1 module is available in the manual "FM 350-1 Function Module" in Entry ID: 1086726. More information about the other function modules is available in the module-specific documentation.
Warning:
Many function modules require special parameterization software which is supplied with the function module and corresponding documentation. Hardware interrupt-triggering events can only be configured together with STEP 7 and this software.

Alarm OBs in SIMATIC S7-300:
In SIMATIC S7-300 CPUs, temporary local data is made available in the alarm OB40. The channel/bit where the hardware interrupt event occurred is specified through this temporary local data.
You can find the description of the alarm OBs in STEP 7 whenever you create the new alarm OB40 in the CPU's block folder (right click > Insert new object > Organization block  > OB40), select the newly created OB40 and then press "F1". This opens the S7 Help for the alarm OB40. Needless to say, if there already is an alarm OB40 present, you can select it straight away and then press "F1".
More information about the module-specific evaluation of the local data is available in the manual "Programmable Logic Controller S7-300 Module Data" in Entry ID 8859629 Chapter 3 (Digital Modules) and Chapter 4 (Analog Modules) or in the special manuals relating to the function modules.

Two alarm OBs (40 and 41) can be used in the CPU 318-2 DP. More information about the CPU 318-2 DP is available in the manual "Programmable Logic Controller S7-300 CPU Data, CPU 312 IFM to CPU 318-2 DP" in Entry ID: 8860591.

There are up to eight alarm OBs available in the S7-400 CPUs. More information about hardware interrupts for S7-400 CPUs is available in Entry ID: 23659324 

Configuring a hardware interrupt:
Hardware interrupts can be configured in the hardware configuration under the properties for the modules with alarm capability.
You can find an example of how to calculate the alarm-response time for the S7-400 in the manual "S7-300 CPU 31xC and CPU 31x, Technical Data" in Entry ID: 12996906 section 5.5 ff.

Configuring hardware interrupt modules:
The modules with hardware interrupt capability can also be configured during an ongoing program cycle by means of system functions SFC 55 (WR_PARM), SFC 56 (WR_DPARM) and SFC57 (PARM_MOD). You can find out how to configure the corresponding data records with the system functions in the manual "Programmable Logic Controller S7-300 Module Data" in Entry ID: 8859629.
Information about the different SFCs is available in the S7 Online Help and in the manual "System Software for S7-300/400 System and Standard Functions" in Entry ID: 1214574 in section 7.1.
Warning:
System functions SFC 55, SFC56 and SFC 57 cannot be used with PROFINET IO.

Note:
Some digital modules (e.g. 6ES7321-7BH0X or 6ES7321-7RD00) require a data length of 6 bytes to write the data record DS1. In the "RECORD" parameter of SFC55 increase the ANY data type by 2 bytes to 6 bytes. The additional 2 bytes must have the value zero.

Blocking, delaying, releasing hardware interrupts:
Hardware interrupts can be blocked, delayed and released again by means of system functions SFC 39 (DIS_IRT IRT_FUNC), SFC 40 (EN_IRT IRT_FUNC), SFC 41 (DIS_AIRT IRT_FUNC) and SFC 42 (EN_AIRT IRT_FUNC).
Information about the different SFCs is available in the S7 Online Help and in the manual "System Software for S7-300/400 System and Standard Functions" in Entry ID: 1214574 in chapter 12.

Description
The maximum number of OPs that a CPU S7-300 can communicate with depends on the connection resources of the CPU concerned in each case. Of the total number of connections there is a default setting always reserving one connection for the PC communication and one for the OP communication. No smaller value can be set. 

You can use the remaining connections for OP communication if you do not need any PG or S7 basic communication connections or are not implementing any CPs that might also occupy "Other Connection" (e.g. CP343-1 for communication with SEND/RECEIVE data lengths >240 bytes).

In many SIMATIC S7-300 CPUs (see table below) you can reserve connection resources for PG, OP, S7 Basic communication and routing.
 

CPU	Order number	Number of connections
312	6ES7312-1AD10-0AB0 6ES7312-1AE1x-0AB0	6
312 C	6ES7312-5BD0x-0AB0 6ES7312-5BE03-0AB0	6
313 C	6ES7313-5BE0x-0AB0 6ES7313-5BF03-0AB0	8
313 C-2DP	6ES7313-6CE0x-0AB0 6ES7313-6CF03-0AB0	8
313 C-2PtP	6ES7313-6BE0x-0AB0 6ES7313-6BF03-0AB0	8
314	6ES7314-1AF1x-0AB0 6ES7314-1AG1x-0AB0	12
314 C-2DP	6ES7314-6CF0x-0AB0 6ES7314-6CG03-0AB0	12
314 C-2PtP	6ES7314-6BF0x-0AB0 6ES7314-6BG03-0AB0	12
315-2DP	6ES7315-2AG10-0AB0 6ES7315-2AH14-0AB0	16
315-2PN/DP	6ES7315-2EG10-0AB0 6ES7315-2EH1x-0AB0	16
315 F-2DP	6ES7315-6FF0x-0AB0	16
315 F-2PN/DP	6ES7315-2FH1x-0AB0 6ES7315-2FJ14-0AB0	16
315 T	6ES7315-6TG10-0AB0 6ES7315-6TH13-0AB0	16
317-2DP	6ES7317-2AJ10-0AB0	32
317-2PN/DP	6ES7317-2EJ10-0AB0 6ES7317-2EK1x-0AB0	32
317 F-2DP	6ES7317-6FF0x-0AB0	32
317 T	6ES7317-6TJ10-0AB0 6ES7317-6TK13-0AB0	32
317 F-2PN/DP	6ES7317-2FJ10-0AB0 6ES7317-2FK1x-0AB0	32
319-3PN/DP	6ES7318-3EL0x-0AB0	32

You can set the connections in the SIMATIC Manager via "HW Config > CPU > Properties > Communication".

Fig. 1: Settings of the connection resources

More information is available by pressing the Help button in the mask above and in the index of the STEP 7 Online Help under "Information on using connection resources".

If you have not reserved the maximum number of connections, it is possible to use all the connections that are still available to make use of the maximum number of connection resources. 

The example in the following figure shows that with the settings made there it is possible to set up a minimum of 4 and maximum of 6 connections to OPs.

Fig. 2: Setting options of the connection resources

The communication services occupy the connection resources in chronological order of the logons. If you attempt to set up more connections than there are resources available, these communication connections are only set up once existing connections have been cleared down and resources are made available again.
Depending on requirements in terms of response times, there are ways of increasing the number of connections:

• Use a CPU with greater resources.
• Use one or more CP 342-5s.
  It makes sense to connect several OPs to a CP because each CP occupies a connection.

S7 connections with PUT/GET functions do not occupy any connection resources, even if you have set up the connections earlier.

Note

• More information on the connection resources of the CPU you are using is available in the device manual "S7-300, CPU 31xC and CPU 31x: Technical Data" in Entry ID:12996906.
• General information on communication with SIMATIC is available in the manual "Communication with SIMATIC" (Entry D 1254686).

Description:
If the overall power requirement of the S7-300 modules in subrack 0 exceeds the power supply provided by the CPU on the backplane bus, then you must implement interface modules.
The S7-300 CPUs provide 1.2 A for the backplane bus. If this is not sufficient, then you must implement interface modules IM 360 /IM 361. If you implement IM 360 (requires 350 mA itself), then there is only 800 ma left for the 8 possible I/O modules in subrack 0.
Interface modules IM 361 each provide 800 ma for the backplane bus in subracks 1 to 3.

Notes:

• CPUs 312 and 312 C, with which you can only have a single-line setup, provide 800 ma
• IM 365 cannot be used to cover increased power requirements in another subrack, because it can only distribute the power supply provided by the CPU in subrack 0 to subrack 1. IM 365 can distribute max. 800 ma of the power provided by the CPU in subrack 0 to subrack 1. IM 365 itself requires 100 ma from the backplane bus.
   
• You can find more information on this in the manuals:
  • "Programmable Logic Controllers S7-300 Module Data" in Entry ID: 8859629
  • "S7-300, CPU 31xC and CPU 31x: Installation Operating Instructions" in Entry ID 13008499.
     

Configuration Notes:
PROFIBUS DP communication in a master-slave system does not use connection resources as long as data is exchanged exclusively via load and transfer commands. The number of your DP slaves therefore does not detract from your connection resources pool. Nor do you have to take the DP communication into account in the "Communication" tab (available in many SIMATIC S7-300 CPUs) of the CPU Properties dialog in HW Config when reserving resources.

In all other cases such as S7 Basic communication, S7 communication, S5-compatible communication etc., where data exchange is via communication blocks, you must have connection resources available.

Keywords:
Connection

Configuration Notes:
The maximum number of CP modules that can be slotted depends on the CP type, the CPU type and the CPU-specific connection resources that are available. All the details are available in the following documentation:

• Reference manual "Automation System S7-400 Module Specifications" (Entry ID 1117740)
• Device manual "CPU 31xC and CPU 31x, Technical Data" (Entry ID: 12996906)
• SIMATIC PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP (Entry ID 8860591).

In each case in the sections concerning the technical data under the heading "Configuration" (number of CPs) and "Communication" or "Communication functions" (connection resources).

The number of Cps for a point-to-point connection (PtP), for example for the SIMATIC S7-400 CPUs, depends only on the number of connection resources. It is limited to 8 for SIMATIC S7-300 CPUs.

Example:
You want to configure two Ethernet and 16 point-to-point connections in a SIMATIC S7-400 station.

• You need one CP443-1 per Ethernet connection.
• You need one CP441-2 for every two PtP connections.

You need resources for 20 connections. Must also use at least one CPU 414-... that supports 32 connections (CPU 413... only has 16 connection resources).

Keywords:
Communications processor

Description:
Each connection requires connection resources for the end point or for the transition point (e.g. CP) on the stations concerned. The number of connection resources is CPU/CP-specific. The behavior described above indicates a resources bottleneck. You might not even be able to go online at all with your PG. Check the number of existing communication connections. Does you CPU have sufficient connection resources? The relevant information is available in the manuals...

• Automation System S7-400 Module Specifications (Entry ID 1117740)
• CPU 31xC and CPU 31x, Technical Data (Entry ID: 12996906)
• SIMATIC PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP (Entry ID 8860591).

Communication problems with OPs due to lack of connection resources occurred in SIMATIC S7-300 usually with old CPUs, which on the one hand had little connection resources and on the other the connection resources could not yet be reserved for PG, OP and S7 Basic communication. A table of the CPUs with which you can reserve resources is included in Entry ID: 299124. Your problem might well be cleared by reserving. Otherwise you should upgrade to a new CPU firmware version.

The maximum number of breakpoints that can be used is determined by the CPU used.

Four breakpoints:

• All SIMATIC S7-400 CPUs
• All SIMATIC S7-300 CPUs as from firmware V3.x
• CPU 318-2 DP

Two breakpoints:

• All SIMATIC S7-300 CPUs (except CPU 318-2 DP) with firmware lower than V3.x

If you have used up your breakpoint resources, you must first reset (delete) a breakpoint before you can set it at another point. Here you must note that breakpoints already run through continue to occupy resources. If you exceed the maximum number of breakpoints that can be used, you get an error message "D062 / D063" (resources exceeded).