Connecting Optical Fiber Cable to Siemens S7-200 CPU 224XP

1. Overview: The S7-200 Fiber Connectivity Problem

The Siemens SIMATIC S7-200 PLC family — including the CPU 224XP — exposes only electrical RS-485 ports for serial communication. There is no integrated fiber-optic transceiver, no SFP cage, and no optical port on any S7-200 CPU variant. Engineers who need a galvanically isolated, electrically noisy-environment-tolerant, or long-distance link between a PC (master) and an S7-200 (slave) must therefore insert a media converter between the PLC's 9-pin D-sub RS-485 port and the fiber segment. The two factory-blessed options are the SIMATIC NET Optical Bus Terminal (OBT) and the SIMATIC NET Optical Link Module (OLM). Each has different protocol, baud-rate, and topology implications.

This reference covers the electrical interface of the CPU 224XP, the converter hardware (OBT 6GK1500-3AA00 and OLM 6GK1502-2CB10), the supported protocols (PPI, MPI, PROFIBUS DP, Freeport), the plastic optical fiber (POF) cable part numbers, attenuation budgeting, termination, STEP 7 Micro/WIN configuration, and a verification checklist. A troubleshooting matrix and FAQ close the document.

2. S7-200 CPU 224XP Communication Port Architecture

The CPU 224XP (order number 6ES7 214-2AD23-0XB0 and similar) provides two RS-485 ports, designated Port 0 and Port 1, both on 9-pin female D-sub connectors (pin assignments are not standard EIA-232). Port 0 supports the full set of operating modes; Port 1 is restricted.

The RS-485 transceiver is a half-duplex, 2-wire differential bus. The PLC provides internal bias resistors and selectable termination that can be enabled with the cable shield clamps at the connector. Pin assignment follows the PPI convention:

3. Why Direct Fiber Termination Is Not Possible

Three physical and electrical constraints prevent direct fiber termination on the S7-200:

1. No optical transceiver. The CPU 224XP PCB carries a Maxim or equivalent RS-485 line driver, an opto-isolator (Port 0 only), and bias/termination networks — but no LED or photodiode at 650 nm. The host plastic used in the housing is opaque.
2. Single-ended logic supply. The port generates a 5 V rail on pin 6 to power the Siemens bus terminator. A POF transceiver requires 3.3 V or 5 V bias and a 24 V DC supply to drive the optical emitter. No socket exists.
3. Asymmetric baud rate vs. fiber distance. Fiber's main advantages — galvanic isolation, multi-kilometer reach, and immunity to ground potential differences — are wasted inside the cabinet. The S7-200's 0.5 km maximum PPI trunk at 9.6 kbps is rarely the bottleneck; noise and ground loops are. A short copper pigtail plus a converter is the correct topology.

Therefore, every S7-200 fiber installation is a three-segment link: PLC ↔ copper pigtail ↔ media converter ↔ POF cable ↔ media converter ↔ copper pigtail ↔ PC / next PLC. The media converter is a bit-level repeating transceiver that adds 1.5–2.5 bit times of propagation delay per hop.

4. Optical Bus Terminal (OBT) 6GK1500-3AA00

The SIMATIC NET OBT 6GK1500-3AA00 is the lowest-cost, smallest-footprint converter intended for PPI / MPI / PROFIBUS segments using POF. It is a two-channel device: two RS-485 ports (electrical) and two POF ports (optical), with internal repeater logic between the four ports. It can be powered from the PLC's pin 7 (+24 V) and pin 2 (M24V) of the connected CPU, eliminating the need for a separate DIN-rail power supply.

Connecting the OBT is straightforward. Pin 7 (+24 V) of the CPU powers the OBT, pin 2 (M24V) returns ground, and pins 3/8 carry the differential signal. The OBT then re-drives the same bits onto the POF segments. Two OBTs form an electrical-to-optical-to-electrical link:

PC (CP5611 / CP5512 / USB-PPI) --- RS-485 ---> OBT #1 --- POF ---> OBT #2 --- RS-485 ---> S7-200 CPU 224XP Port 0

5. Optical Link Module (OLM) 6GK1502-2CB10

OLM modules are the heavy-duty alternative. They are designed for PROFIBUS DP and MPI networks running at 1.5 Mbps and higher, with line, ring, and redundant topologies. The 6GK1502-2CB10 is the OLM/P11 — one RS-485 electrical channel, one optical channel (two POF strands for full-duplex/loop). It is the right answer when the fiber must span hundreds of meters, the bus must be a redundant ring, or PROFIBUS DP at 1.5 / 3 / 6 / 12 Mbps is in use.

OLM-to-OLM topology forms an electrical bus segment per OLM and an optical segment between OLMs. Multiple OLMs can be daisy-chained with up to 50 m of POF between adjacent OLMs. Ring topology requires that the last OLM's optical port be wired back to the first OLM to provide media redundancy (typical OLM feature, not an S7-200 feature).

6. Freeport vs PPI / MPI / DP Protocol Implications

The choice of protocol on the CPU 224XP's RS-485 port determines what the converter must do at the bit level and what the PC application must do at the protocol level.

Freeport mode on the S7-200 is a user-controlled UART mode (XMT/RCV instructions in the program). The byte stream is identical to a stock RS-485 line: 1 start, 8 data, 1 stop, no parity (programmable). Because the OBT and OLM are physical-layer devices and do not parse frames, they do not care whether the bytes are PPI, Modbus RTU, or a custom ASCII protocol. The propagation delay of the OBT (~1.5 bit times per hop) and the OLM (~2.5 bit times per hop) are well within the half-bit timing tolerance of any standard protocol at 115.2 kbps and below.

For long fiber segments using Freeport at 115.2 kbps, verify that the round-trip optical delay (2 hops × ~5 ns per meter × 100 m = 1 µs) plus converter delay stays below the frame turnaround. The S7-200 Freeport turnaround is the XMT/RCV inter-message gap, which is fully user-defined and therefore tolerant.

7. Plastic Optical Fiber Cable and Connector Specifications

For an in-cabinet or inter-cabinet POF link, the Siemens pre-assembled patch cables are the lowest-risk option. The 6XV830-1 family uses 980 µm core, 1 mm outer jacket, with HP/PF (Siemens designation) push-pull connectors on both ends.

POF attenuation at 650 nm is typically 0.2 dB/m. The optical power budget of the OBT and OLM is approximately 11 dB, giving a maximum segment length:

L_max = Power budget / attenuation per meter
L_max = 11 dB / 0.2 dB per m = 55 m

The de-facto Siemens limit is 50 m per POF segment, leaving 1 dB margin for connector losses and aging. For runs beyond 50 m, step up to PCF (plastic-clad silica, 200/230 µm) or glass (62.5/125 µm) using OLM/G11 (6GK1503 series) instead. Splicing raw POF on site requires a special crimp tool and is not recommended for production lines.

8. Topology, Segment Lengths, and Termination

The S7-200 PPI/MPI segment rules apply unchanged to the electrical stubs and to each electrical segment between two OBTs. The OBT itself is a repeater, so the POF segment counts as one bus segment. Termination must be enabled only at the two physical ends of any electrical segment, never in the middle.

Two segments are formed — the electrical segment A between the PC and OBT #1, and the electrical segment B between OBT #2 and the S7-200. The POF span is a third segment only at the optical layer. Termination is enabled on segment A at the PC, on segment B at the S7-200, and not at the OBTs (the OBT contains no bus terminator network for the cable it repeats into).

9. Commissioning Step-by-Step Procedure

Use the following order to bring up a PC-to-S7-200 fiber link through the OBT.

1. Power down both devices. Disconnect the S7-200 from mains and the PC from its bus. Fiber cable is not energized; this step is for the electrical segments.
2. Wire the PC end. Connect CP5611 (PCI) or CP5512 (PCMCIA) or USB-PPI adapter Port 0 to OBT #1's RS-485 port using a 9-pin D-sub cable, pins 3/8 (data) and 2 (ground) only. Enable 220 Ω termination at the PC end (DIP switch on CP, or jumper on the USB-PPI).
3. Wire the PLC end. Connect the S7-200 Port 0 to OBT #2's RS-485 port. Enable termination on the S7-200 via the cable connector switch or by shorting pins 3–8 with 220 Ω if the bus terminator is not present. Connect +24 V from PLC pin 7 to OBT #2 +24 V terminal; M24V (pin 2) to OBT ground terminal.
4. Pull the POF patch. Lay cable 6XV830-1CH15 or 6XV830-1CH30 between OBT #1 optical port and OBT #2 optical port. Lock the HP/PF connectors with their built-in latches; the connectors click when seated.
5. Power up. Energize the S7-200 first, then the PC. Verify OBT green power LED and per-port TxD/RxD indicators.
6. Configure the PC adapter. In STEP 7 Micro/WIN, open Communications → Set PG/PC Interface. Select the installed CP and set PPI baud rate to 9.6 kbps (default for S7-200) or 19.2 kbps if the S7-200 system block has been changed. Set highest station address to 31 and transmission delay to 0.
7. Configure the S7-200 PPI address. In Micro/WIN, open System Block → Communication Ports. Set Port 0 address (default 2) and baud rate. Download the system block. The S7-200 enters RUN if the user program is valid.
8. Test the link. In Micro/WIN, click Communications → Search for S7-200 CPUs. The S7-200 should appear with its address, baud rate, and CPU type. Double-click the entry to go on-line.

10. Verification, Diagnostics, and Fault Mapping

After commissioning, validate the link with three checks. They map to specific failure modes and pinpoint the failing segment.

Use the S7-200 ERR LED as the single best indicator. If ERR is illuminated on a port that you have just converted to fiber, the link is electrically present (the OBT is sending bits) but the baud rate or bit timing is wrong. Lower the baud rate, retest, and only then consider replacement of the OBT.

11. Alternative Architectures

When the OBT does not fit, evaluate these substitutes in order of preference.

12. Frequently Asked Questions