Configuring SINEC L1 Communication with Siemens S5 PLCs

Configuring SINEC L1 Communication with Siemens S5 PLCs

Siemens SINEC L1 is a legacy RS-485-based fieldbus designed for the SIMATIC S5 family. It is the lower layer in Siemens' SINEC hierarchy, sitting beneath SINEC H1 (10 Mbit/s industrial Ethernet) and later SINEC L2 (Profibus). S5-100U and S5-115U systems deployed in the 1980s and 1990s frequently use SINEC L1 for master/slave data exchange, but the bus is undocumented in modern Siemens portals and is treated as legacy. This reference consolidates the constraints that govern SINEC L1 wiring, addressing, and migration to S7-1200 or S7-1500 systems.

SINEC L1 vs. SINEC H1: Architectural Position

SINEC L1 and SINEC H1 share the Siemens SIMATIC networking heritage but address different layers:

Engineers commissioning mixed-vintage plants must distinguish the two buses clearly: a CPU 103 with a SINEC L1 slave port is not addressable over SINEC H1 without a router CP (e.g., CP 1430 with the L1 gateway firmware option). Verify CP capabilities against the printed Siemens CP manual before assuming cross-bus transparency.

S5-100U CPU 103 Compatibility on SINEC L1

The S5-100U family is the smallest member of the S5 line and uses a CPU-integrated sub-D connector for PG/OP and SINEC L1 access. The CPU 103 (and its variants CPU 102, CPU 103-3, CPU 104) exposes a 15-pin sub-D port labeled for programming, OP, and L1 bus operation.

For SINEC L1 deployment of a CPU 103, three conditions must hold:

1. The CPU sub-D connector must be physically free (no PG cable or OP permanently attached on the same port in master mode).
2. The CPU must be configured as an L1 slave when using the integrated port. Master operation on a CPU 103 typically requires a plug-in CP 521 or CP 530 module, which is rare in S5-100U chassis because of slot limitations.
3. The BT 777 bus terminal must be wired between the L1 trunk cable and the CPU sub-D port. The BT 777 is the level converter / bus terminator; it converts the 24 V RS-485 levels of the trunk to the TTL levels expected at the CPU.

Reference: S5-100 system manual, Chapter 13 ("Raccordement des automates au câble du bus L1"). The procedure described in the manual is:

1. Connect the SINEC L1 trunk cable to the BT 777 terminal.
2. Connect the BT 777 drop cable to the PG/OP/SINEC L1 sub-D connector on the CPU front panel.
3. Verify termination: 150 Ω line terminators (or the BT 777 internal termination if provided) at both physical ends of the L1 segment.

S5-115U SINEC L1 Compatibility

The S5-115U is a modular mid-range PLC with a backplane that accepts dedicated communications processors. S5-115U deployments almost always use a plug-in CP (CP 521 SI, CP 521 BASIC, CP 530, or CP 1430 for H1) for L1 traffic. Direct CPU-front-panel L1 access on the S5-115U central controller is generally not used; L1 traffic flows through the CP slot and the CP's own front connector.

Compatibility matrix (verify against the specific CPU and CP order numbers in your plant):

When interconnecting an S5-100U CPU 103 (slave) with an S5-115U CP 530 (master), the same BT 777 trunk rules apply on the 100U side; the 115U side uses the CP 530's own bus connector and termination. Pinout, shield grounding, and segment length budgets must be calculated jointly.

BT 777 Bus Terminal: Physical Layer Requirements

The BT 777 is the canonical SINEC L1 drop terminal. It performs three functions:

1. Level conversion between RS-485 trunk levels (typically ±5 V differential) and the TTL interface of the CPU/CP port.
2. Trunk isolation so a single CPU swap does not collapse the trunk.
3. Strain relief and shield-termination landing for the trunk cable screen.

Mechanical and electrical specifications for the BT 777 should be cross-checked against the printed Siemens BT 777 data sheet, since the values below are typical but not authoritative:

SINEC L1 Topology: Master/Slave Point-to-Point Constraint

A defining constraint of SINEC L1 is that a single CP (or CPU 103 integrated port) acts as either master or slave, and addresses exactly one peer per logical channel. This is fundamentally different from multi-drop Profibus or multi-master Ethernet. Engineers migrating logic from SINEC L1 to Profinet or EtherNet/IP must rewrite polling loops because the new networks allow many slaves per master port.

Implications:

• You cannot connect multiple PLCs through a single CPU 103 sub-D port and expect parallel polling; the port is a point-to-point channel.
• For N slaves, the master must either own N CPs (one per slave channel) or use a CP that supports a polled table of slave addresses (CP 530 in master mode, with explicit configuration of slave addresses 1..30).
• Adding a new slave to an existing L1 segment requires both a free trunk tap and a free master CP channel or a re-allocation of slave addresses.

SINEC H1 Ethernet Path: FBs and OBs

SINEC H1 (10 Mbit/s industrial Ethernet, predecessor to Profinet) was used in larger S5 systems and survived into S7-300/400 deployments via the CP 1430 and CP 1613 modules. SEND/RECEIVE on SINEC H1 is implemented through a fixed block library that engineers must call from the S5 user program:

Typical SEND call from an S5-115U program to a remote S7-400 over H1 (illustrative syntax — verify in STEP 5 documentation):

// S5 STL - SEND via FB 120
JU FB 120
NAME : SEND
SSNR  : KY 0,2     // CP number 0, job number 2
A-NR  : KY 0,0     // AG number / job type
ZTYP  : KS "DR"    // data record type
DBNR  : KY 0,42    // source DB number
ANZW  : MW 200     // status word
QANF  : MW 210     // source start offset
QLAE  : MW 212     // source length
PAFE  : MB 220     // error byte

For cross-vintage plants where a S7-400 must talk to a legacy S5 over H1, the S7-400 side uses the SINEC H1 capability of its CP 1430 (in S7-400 firmware revisions that still ship the H1 protocol stack). Newer CP 443-1 modules drop H1 in favor of Profinet.

SCADA Integration via SINEC H1 (IGSS32 Driver)

For SCADA connectivity to legacy S5 (and S7 with H1 boards), Schneider Electric's IGSS32 ships a dedicated Siemens Sinec H1 communication interface driver. This driver permits a modern SCADA workstation to read/write data on Simatic S5 and S7 PLCs equipped with H1 interface boards without requiring a Siemens engineering station on the segment.

Key parameters (verify in the IGSS32 driver release notes):

• PLC families supported: S5-115U, S5-135U, S5-155U (with CP 143 or CP 1430), S7-300/S7-400 (with H1-capable CP).
• Transport: SINEC H1 frames, ISO transport (RFC 1006 subset).
• Polling: configurable per tag; default block sizes in the 16-256 byte range.

See the official product page for the driver and its current revision status: Schneider Electric IGSS32 - Siemens Sinec H1 driver.

Migration Path: S5-100U / S5-115U to S7-1200

The strongest recommendation for any active SINEC L1 segment is a controlled migration to S7-1200 (or S7-1500) over a planned window. The drivers are economic, availability, and operability — modern spare parts, current TIA Portal tooling, integrated Profinet, and a single programming environment.

Hardware substitution

Software migration

STEP 5 programs cannot be automatically converted to STEP 7 / TIA Portal. Engineers must:

1. Reverse-engineer the S5 program from a printed listing or current PG dump (STEP 5 → File → Save to external).
2. Translate each function block to SCL or ladder (TIA Portal supports FBD/LAD/SCL; structured text is preferred for complex logic).
3. Replace SINEC L1 SEND/RECEIVE with PUT / GET instructions for S7-to-S7 or with Profinet I/O for distributed I/O.
4. Re-map DB and flag addresses; S7-1200 data blocks have a different syntax than S5 DBs.
5. If EPROMs were used in the original S5, plan an EPROM eraser and EPROM programmer — modern TIA Portal does not program S5 EPROMs.

Communication bridging (interim)

If both old S5 and new S7 must coexist during migration, bridge options include:

• SINEC L1 / Modbus RTU gateway — third-party gateways that terminate L1 on one side and present Modbus RTU/TCP on the other. Validate against the S5 slave protocol.
• SINEC H1 to Profinet router — CP 1430 TF paired with an S7-1500 acting as a router; only viable where the S5 side is already H1.
• Soft PLC on the S5 side — replaces the L1 master with a software controller that emulates the S5 L1 traffic. Niche, expensive.

Commissioning Verification Checklist

After wiring and parameterization, verify each segment with these steps:

1. Bus termination. Measure resistance across the trunk with the master and all slaves powered off. Expected reading: ≈ 60 Ω (two 150 Ω terminators in parallel, with fail-safe bias excluded).
2. Shield bonding. Confirm shield is bonded to ground at one end only (or through a capacitor to avoid ground loops).
3. BT 777 supply. Verify 24 V DC at each terminal under load; voltage drop on long drops indicates undersized conductors.
4. Slave address uniqueness. Each L1 slave must have a unique 5-bit address (0-30). Duplicate addresses manifest as silent polling failures, not error codes.
5. CP status LEDs. CP 530 / CP 521 displays a "bus active" indicator. A steady-busy or no-flash state indicates a wiring or addressing fault.
6. Send/receive handshake. Trigger FB 120 SEND from the master; confirm ANZW (status word) transitions to a known-good code per the CP manual.
7. OB 230-237 monitoring. If a CP is wired, monitor these error OBs on the master for the first commissioning run; record any non-zero event codes.

Troubleshooting Matrix

Field-Proven Caveats

• The CPU 103 integrated sub-D port cannot be a master for more than one slave without external CP support — repeated attempts to multi-drop through this port waste commissioning time.
• SINEC L1 documentation is in French and German historical manuals; Siemens' current support portal de-prioritizes L1 in favor of Profibus/Profinet. Preserve a local PDF copy of the S5-100 Chapter 13 document.
• BT 777 stock is exhausted at most distributors. New builds should not be planned around SINEC L1.
• EPROM-based S5 programs require a UV eraser and a programmer that supports Siemens S5 EPROM format. TIA Portal cannot read S5 EPROMs.
• SCADA integration via SINEC H1 (e.g., IGSS32) depends on the CP's H1 firmware option being licensed and active; not all CP 1430 modules ship with H1 enabled.
• Cross-bus addressing between SINEC L1 and SINEC H1 requires a router CP — these are not the same physical bus and cannot be wire-joined.

FAQ