Locating and Licensing Redundant Modbus TCP Function Blocks (FB 516 and FB 510) in SIMATIC PCS 7

In SIMATIC PCS 7 systems, function blocks FB 516 (MODTCP_COMRED) and FB 510 (MODTCP_COMRED_T) are not included in the standard PCS 7 DVD installation or basic library media. These blocks belong to specialized Siemens Modbus/TCP Redundant communication packages designed for high-availability controllers communicating with third-party systems via CP 443-1 modules or integrated PROFINET interfaces. Deploying these blocks requires purchasing the dedicated add-on software package and registering a runtime license key per automation system with an authorized Siemens distributor.

Modbus TCP Redundant Communication Architecture in PCS 7

High-availability SIMATIC PCS 7 controllers (S7-400H and CPU 410-5H) frequently interface with third-party distributed control systems, substations, and supervisory platforms using Modbus TCP. Because redundant controller architectures feature dual CPUs and redundant Ethernet connections, standard single-channel communication drivers cannot manage automatic connection switchover during controller failover or line breakage.

To address high-availability requirements, Siemens developed dedicated redundant Modbus TCP libraries. These blocks manage dual TCP connection endpoints, perform health heartbeats, coordinate master/standby communication roles, and execute bumpless failover between primary and secondary communication paths without dropping active process data exchanges.

Differentiating FB 516 (CP 443-1) and FB 510 (PN-CPU)

The specific function block required depends on the controller hardware and network interface topology:

FB 516 (MODTCP_COMRED):
Utilized on S7-400 / S7-400H systems where Ethernet communication is handled by external communications processors (CP 443-1, e.g., 6GK7443-1EX30-0XE0).
Executes data exchange using standard communication function blocks (AG_SEND / AG_RECV or BSEND / BRECV).
Manages dual CP configurations where each rack of the redundant H-system contains an independent CP 443-1 card.
FB 510 (MODTCP_COMRED_T):
Utilized on controllers featuring integrated PROFINET/Ethernet interfaces (such as CPU 410-5H or PN-enabled S7-400 CPUs).
Executes data exchange using Open User Communication (TCON, TSEND, TRCV, TDISCON).
Bypasses external CP cards, executing high-speed protocol processing directly within the controller's integrated Ethernet subsystem.

Procurement, Licensing, and Software Entitlements

Because FB 516 and FB 510 are specialized engineering add-ons, they cannot be downloaded freely from the Siemens Industry Online Support (SIOS) public download area:

Commercial Add-On Package: The engineering library must be procured as a licensed software product through standard Siemens sales channels.
Runtime Licensing Model:
- The software package includes an engineering library and a Certificate of License (CoL).
- For redundant systems, a redundant license pair must be procured to cover both controller CPUs.
Runtime Registration (REG_KEY):
- During commissioning, the software generates a registration query code based on the CPU hardware serial number.
- The engineer submits this code to the Siemens licensing portal to obtain a 17-character alpha-numeric unlock key (REG_KEY).
- Entering this key into the block's registration data block permanently clears demo mode and prevents communication timeouts.

Modbus TCP Redundant Software Package Matrix

The following table details the Siemens software packages for redundant Modbus TCP communication:

Library Function BlockInterface TypeApplicable HardwareSiemens Software PackageTypical MLFB / Package CodeFB 516 (MODTCP_COMRED)External Ethernet CPS7-400H + Dual CP 443-1SIMATIC Modbus/TCP CP Redundant2XV9450-1MB11FB 510 (MODTCP_COMRED_T)Integrated PN PortsCPU 410-5H / S7-400 PNSIMATIC Modbus/TCP PN-CPU Redundant6AV6676-6YM10-0AX0Single FB 102 (MB_PNCL)Integrated PN PortSingle S7-410 / S7-400SIMATIC Modbus/TCP PN-CPU Single2XV9450-1MB02Single FB 100 (MODBUSCP)External Ethernet CPSingle S7-400 + CP 443-1SIMATIC Modbus/TCP CP Single2XV9450-1MB00

Commercial Accuracy Disclaimer: Software order numbers (MLFBs), package structures, and firmware compatibility prerequisites vary across PCS 7 versions and hardware platforms. System integrators must confirm exact software entitlements and licensing requirements against the official Siemens Catalog CA 01 and obtain an authorized Siemens distributor quotation prior to purchasing.

Verification

Verify redundant Modbus TCP block deployment:
1. In CFC, open the chart containing FB 516 or FB 510 and verify that INIT = True completes without block configuration errors.
2. Inspect output parameter LICENSED; verify it transitions to True after parameterizing the valid REG_KEY.
3. Verify that the block output STATUS reads 16#0000 (connection established and idle ready).
4. Simulate a primary Ethernet cable disconnect; confirm that communication switches to the secondary connection within the configured failover time without CPU fault or buffer overflow.

Limitations and Operational Precautions

Essential technical and licensing rules:
- Unlicensed blocks operate in demo mode for a limited evaluation period (typically 2 hours), after which communication halts with status 16#A090 until the CPU is restarted.
- Never mix CP-based blocks (FB 516) with integrated PN-CPU blocks (FB 510); doing so creates parameter type collisions in NetPro and connection table corruption.
- Ensure that the cycle execution time of the cyclic interrupt OB hosting the Modbus block (e.g., OB35 at 100 ms) provides sufficient CPU processing margin under maximum communication load.

Engineer FAQ

Can I download FB 516 or FB 510 for free from SIOS?

No. These blocks are part of commercial add-on software packages that require official purchase and license entitlement. SIOS provides application descriptions and documentation, but the block libraries require a purchased license.

What happens if the REG_KEY is missing or invalid?

The block functions in demo mode for approximately two hours, after which outputs lock and communication stops with an error status. The controller must be restarted to reset demo mode.

Does one license cover both CPUs in a redundant S7-400H system?

The redundant software package (such as 2XV9450-1MB11 or 6AV6676-6YM10-0AX0) provides licensing entitlements specifically designed for redundant CPU pairs.