1. Problem Overview
The Omron C200H-LK201 Host Link Unit is the serial communication module used to bridge a SYSMAC C200H (and C200HS / C200Hα) rack to a host computer running CX-Programmer (or the legacy SYSMAC Support Software). When the unit is installed on a C200H-CPU01 (or CPU02/CPU03/CPU11/CPU21/CPU22/CPU23) and connected to a PC over RS-232C, the most common field failure presents as follows:
- The RUN LED on the LK201 front bezel does not illuminate.
- Pressing the Online Work – Connect command in CX-Programmer causes the RCV LED to flicker briefly, but the XMT and ERROR LEDs never light.
- CX-Programmer raises an "Unable to connect to PLC" or "Communication error" dialog and aborts the session.
- The PLC itself continues to run normally (no CPU error, no I/O bus fault).
This symptom cluster is one of the most frequently reported issues on legacy C200H installations and almost always traces to one of three root causes: a seating or connector fault between the LK201 and the CPU/backplane, a PLC reset condition that prevents the unit from initializing, or a defective Host Link Unit. The remainder of this article walks through a deterministic field procedure to isolate and correct each cause.
2. Affected Hardware, Firmware and Catalog Numbers
| Item | Catalog / Part Number | Notes |
|---|---|---|
| CPU | C200H-CPU01 (also -CPU02, -CPU03, -CPU11, -CPU21, -CPU22, -CPU23) | All C200H-family CPUs share the same I/O bus; LK201 must mount on a C200H-BCxx Backplane. |
| Host Link Unit (original) | C200H-LK201 | Supports Host Link (C-mode) protocol only; max 32 nodes on RS-422 multidrop. |
| Host Link Unit (enhanced) | C200H-LK201-V1 | Adds 1:N protocol extension, higher command throughput, and improved error counters. Drop-in replacement on the same backplane slot. |
| RS-232C cable | Omron XW2Z-200P (2 m) / XW2Z-500P (5 m) | Pre-built, DB9 female to DB25 male, wired per table below. |
| Programming software | CX-Programmer v9.x (or SSS / CPT for legacy users) | Set Host Link driver, not SYSMAC WAY or Toolbus. |
| Operating system | Windows XP / 7 / 10 (with USB-to-serial adapter if needed) | Verify the COM port in Device Manager matches CX-Programmer setting. |
The LK201 and LK201-V1 are electrically and mechanically interchangeable on a C200H rack. The -V1 suffix only changes internal firmware revision; both respond to the same DIP switch assignments and present the same DB25 pinout on the front bezel.
3. RS-232C Cable Wiring Specification (DB9 ↔ DB25)
The cable between the PC and the LK201 must follow the Omron Host Link 1:1 RS-232C wiring shown below. Note that the LK201's DB25 port is wired as DCE from the PLC's point of view, so a straight-through mapping on pins 2, 3 and 5 — combined with handshake loopback on the PC side — produces a correct crossover when viewed from either end.
| PC DB9 (DTE) | Signal | LK201 DB25 (DCE) | Signal | Direction |
|---|---|---|---|---|
| 1 (DCD) | DCD | 8 (CD) | Data Carrier Detect | LK201 → PC |
| 2 (RXD) | Receive Data | 2 (SD) | Send Data | LK201 → PC |
| 3 (TXD) | Transmit Data | 3 (RD) | Receive Data | PC → LK201 |
| 4 (DTR) | Data Terminal Ready | 20 (ER) | Equipment Ready | PC → LK201 |
| 5 (GND) | Signal Ground | 7 (SG) | Signal Ground | — |
| 6 (DSR) | Data Set Ready | 6 (DR) | Data Set Ready | LK201 → PC |
| 7 (RTS) | Request to Send | 4 (RS) | Request to Send | PC → LK201 |
| 8 (CTS) | Clear to Send | 5 (CS) | Clear to Send | LK201 → PC |
| 9 (RI) | Ring Indicator | 22 (CI) | Call Indicator | LK201 → PC |
Required loopback jumpers (soldered on the DB9 connector shell at the PC end):
- DB9 pin 4 (DTR) → DB9 pin 6 (DSR)
- DB9 pin 7 (RTS) → DB9 pin 8 (CTS)
These jumpers satisfy the LK201's DR/CS handshake by echoing the PC's outgoing control signals back to itself. Without them, CX-Programmer will time out waiting for a CTS or DSR response.
4. LK201 Front Panel Switch Configuration
The LK201 carries four rotary DIP switches behind a small cover on the front face. Each switch has 16 positions (0–F). Correct settings for a 1:1 Host Link session with CX-Programmer are:
| Switch | Function | Position | Field Value | Comment |
|---|---|---|---|---|
| SW1 | Host Link unit number | 0 | Node 0 | Must match CX-Programmer "Network/Unit Number". Valid range 0–31 for -V1, 0–F for original LK201. |
| SW2 | Reserved / I/O response delay | 0 | Default | Position 0 leaves I/O scan at minimum; positions 1–F extend the response delay in 10 ms increments. |
| SW3 | Baud rate | 5 | 9 600 bps | Decoded in table below. |
| SW4 | Command level / link mode | 2 | Host Link (1:1 mode) | Position 1 = 1:N Host Link, Position 2 = 1:1 Host Link, Position 3 = 1:1 Toolbus. |
SW3 — Baud rate decode (LK201 / LK201-V1):
| SW3 | Baud (bps) | SW3 | Baud (bps) |
|---|---|---|---|
| 0 | 300 | 5 | 9 600 (recommended default) |
| 1 | 600 | 6 | 19 200 (-V1 only) |
| 2 | 1 200 | 7 | Reserved |
| 3 | 2 400 | 8 | Reserved |
| 4 | 4 800 | 9 | Reserved |
The factory-default SW4 = 2 setting places the unit into 1:1 Host Link mode — this is what CX-Programmer's "Host Link" driver expects. If SW4 is accidentally set to 3 (Toolbus), CX-Programmer's Host Link driver cannot establish a session and the LK201 will silently reject every frame.
5. LK201 Rear DIP Switch Configuration
On the rear face of the LK201 — visible after the unit is slid out of the backplane — is a four-position slide DIP that controls physical-layer options. For a single LK201 connected directly to a PC over RS-232C, configure as follows:
| Rear Switch | Function | Position | Reason |
|---|---|---|---|
| 1 | RS-422 line terminator (120 Ω) | OFF | Terminator only required on last node of an RS-422 multidrop. Single RS-232C node → OFF. |
| 2 | 4-wire / 2-wire RS-422 selector | OFF | Don't care for RS-232C link. |
| 3 | RS-232C / RS-422 link selector | ON | Routes the front DB25 to the RS-232C transceiver. Must be ON for a PC connection. |
| 4 | Echo / response mode | OFF | Standard Host Link echo behaviour for 1:1 sessions. |
6. CX-Programmer Communication Settings
In CX-Programmer, create or open the project for the C200H-CPU01, then open PLC → Auto Online → Direct Online or right-click the new PLC in the project tree and choose Change Connection Method. The dialog Host Link Settings must match the LK201 front panel exactly:
| CX-Programmer Field | Required Value | Matches LK201 |
|---|---|---|
| Network Type | Host Link | — |
| COM Port | COM1 (or the actual COM assigned in Device Manager) | — |
| Baud Rate | 9 600 bps | SW3 = 5 |
| Data Bits | 7 | Host Link frame format |
| Parity | Even | Host Link frame format |
| Stop Bits | 2 | Host Link frame format |
| Unit Number | 0 | SW1 = 0 |
| Response Timeout | 2 s (default; raise to 4–5 s over slow links) | — |
The 7-E-2 frame is mandatory for Host Link. CX-Programmer's "Auto Detect" will sometimes negotiate 8-N-1 if the wrong driver is chosen — that frame is rejected by the LK201 and produces the symptom described in section 1.
7. LK201 LED Diagnostic Matrix
The LK201 front bezel has four status LEDs. Each combination maps to a specific subsystem fault. Use the table below as a primary triage tool before touching switches or cables.
| RUN | RCV | XMT | ERROR | Interpretation | First Action |
|---|---|---|---|---|---|
| OFF | blinks | OFF | OFF | LK201 not initialised. Connector, PLC reset, or hardware fault. | Power-cycle PLC. Reseat LK201. |
| ON | blinks | OFF | OFF | PC transmits, LK201 receives, but PC never sees a reply. | Check cable swap, unit number, CX-Programmer driver. |
| ON | blinks | blinks | OFF | Healthy Host Link session. | No action — proceed. |
| ON | blinks | blinks | lit | Framing error or unsupported command. | Verify 7-E-2, unit number, command level (SW4). |
| blinks | OFF | OFF | OFF | Backplane fault or address conflict. | Try another slot; verify CPU I/O table. |
| OFF | OFF | OFF | OFF | No power to LK201. | Check backplane 5 V and backplane connector. |
8. Root Cause Analysis: Why the RUN LED Does Not Light
Per W143-E1-6 §5-4, the "RUN Indicator does not light" condition has three documented causes. The discussion below maps each to a verifiable field check.
8.1 Cause A — Faulty connector contact between PLC and LK201
The LK201 mates with the backplane via the same 96-pin connector as any C200H I/O module. Oxidation, vibration, or a half-inserted seating interrupts the 5 V supply rail and the backplane handshake lines. Symptoms: RUN LED off, no CX-Programmer response, but other I/O modules on the rack continue to scan.
Verify: Power down the PLC completely (remove AC to the C200H-PS211 or equivalent power supply, wait 30 s for capacitor discharge). Depress the LK201 mounting screws fully, then press the unit firmly into the backplane. Re-tighten screws. Restore power.
8.2 Cause B — PLC is resetting the LK201
Certain severe CPU faults (watchdog time-out, memory parity error on the C200H-CPU01) cause the CPU to repeatedly reset its I/O bus. Each reset pulls the LK201's INIT line low, which in turn suppresses the RUN LED. The PLC will display ALARM (red LED) on its CPU front face, or may show a flashing RUN/ALARM pattern.
Verify: Read the C200H CPU's error code by setting the mode selector to MONITOR or PROGRAM and observing the 7-segment display (if present on CPU02/CPU03/CPU21/CPU22) or the ALARM LED blink code on CPU01/CPU11. Typical reset-causing faults:
| CPU Error | Cause | Remedy |
|---|---|---|
| F0 / ALARM steady | Watchdog timer expired | Scan time too long; reduce cyclic task or split program. |
| F1 | CPU bus fault (defective module) | Remove I/O modules one at a time to isolate. |
| F2 | Memory parity | Battery low; replace C200H-BAT09 and re-transfer program. |
| F6 / continuous reset | I/O bus overload | Check for duplicate I/O addresses. |
8.3 Cause C — Defective Host Link Unit
If Causes A and B are eliminated, the LK201 itself is suspect. The most reliable field test is to swap the LK201 with a known-good unit of the same catalog number. Original C200H-LK201 units manufactured before 1995 are now beyond published component-life milestones and a hardware failure should be expected. The C200H-LK201-V1 is a recommended drop-in replacement — it shares the same backplane footprint, the same DIP switch assignments, and the same Host Link command set.
9. LK201 vs LK201-V1 — Practical Differences
| Feature | C200H-LK201 | C200H-LK201-V1 |
|---|---|---|
| Host Link command set | C-mode (legacy) | C-mode + extended 1:N |
| Maximum unit number | 0–F (15) | 0–1F (31) |
| Maximum baud rate | 9 600 bps | 19 200 bps |
| Multidrop RS-422 nodes | Up to 32 | Up to 32 (improved contention handling) |
| LED behaviour | Same | Same |
| DIP switch meaning | Same | Same |
From the technician's perspective the two units are indistinguishable for a 1:1 RS-232C link to CX-Programmer. If you have a spare LK201-V1 in stock, swap it in first — many sites find that the original LK201 has simply reached end of life.
10. Step-by-Step Resolution Procedure
- Verify the cable electrically. With the LK201 disconnected, use a multimeter on continuity mode to confirm 2↔2, 3↔3, 5↔7, and the PC-side DTR/DSR and RTS/CTS jumpers. Replace the cable if any pin is open.
- Power down and reseat. Remove AC from the C200H power supply. Wait 30 s. Press the LK201 firmly into its slot and tighten both mounting screws (0.6 N·m typical).
- Inspect the rear DIP. Slide the unit out and confirm rear SW3 = ON (RS-232C), SW1 = OFF (no RS-422 terminator). Re-insert.
- Restore power. Observe the LK201 front RUN LED. It should light within 2 s of CPU RUN or PROGRAM mode entry.
- Check the CPU. If RUN still does not light, read the CPU error code and resolve using the table in §8.2.
- Verify front DIPs. SW1 = 0, SW3 = 5 (9 600), SW4 = 2 (1:1 Host Link).
- Configure CX-Programmer. Set Network = Host Link, COM port, 9 600 / 7 / Even / 2, unit number 0.
- Attempt connect. In CX-Programmer choose PLC → Online. The RCV LED should flicker continuously, XMT should flicker as the LK201 replies, and the project should enter Online within 3–5 s.
- If still failing, swap the LK201. Move the suspect LK201 to a known-good C200H rack and test the swap unit on the original rack to localise the fault.
- Capture and document. Record the front/rear switch positions, CX-Programmer driver, cable serial number, and CPU error code. File with the site's PLC asset record.
11. Verification and Commissioning Tests
After a successful online connection, perform these four checks before releasing the panel back to operations:
| Test | Action | Pass Criterion |
|---|---|---|
| Watchdog | In CX-Programmer, force and unforce a single IR bit (e.g., IR 000.00). | PLC input reflects the force within one scan. |
| Block read | PLC → Transfer → From PLC → Program/Memory. | Program upload completes without timeout. |
| Cyclic scan | Leave online for ≥ 30 min and observe CX-Programmer's "PLC Scan Time" value. | Scan time stable to ±2 ms. |
| LED health | During the 30 min window, record RUN / RCV / XMT / ERROR duty cycles. | ERROR remains dark; RUN steady ON. |
A session that remains error-free for 30 minutes and survives a PLC mode toggle (PROGRAM → MONITOR → RUN) is considered stable for production release.
12. Common Pitfalls and Field Tips
- USB-to-serial adapters. Many PC serial ports are now USB bridges (FTDI FT232, CH340, CP2102). These introduce a 5–15 ms latency spike on every response. Increase CX-Programmer's response timeout to 4–5 s if RCV flashes but XMT never replies.
- Wrong driver selected. CX-Programmer offers "Host Link", "SYSMAC WAY", "Toolbus", and "Ethernet (FINS/UDP)". Only "Host Link" works against an LK201 over RS-232C.
- Multidrop terminator left ON. On a 1:1 RS-232C link, terminator OFF (rear SW1). A 120 Ω terminator on the RS-422 driver creates a DC bias that prevents the RS-232C transceiver from asserting.
- RS-232C / RS-422 mix-up. Some sites have wired the DB25 for an RS-422 multidrop, leaving rear SW3 OFF. Confirm by tracing the cable shield back to the LK201 DB25 pin 1 — it must be the only shield ground on the segment.
- C200H-CPU01 specific. The CPU01 (early version) does not support the -V1's 19 200 bps setting. If you swap in an LK201-V1 and set SW3 = 6, expect transmission errors. Stay on SW3 = 5.
- Replacement battery. If the C200H-BAT09 is older than 5 years, replace it before long commissioning sessions. A dying battery can corrupt the I/O table during a brown-out and lock the LK201 into a perpetual reset state that mimics a hardware fault.
13. Related Standards and Reference Documents
- Omron W143-E1-6 — Host Link Units System Manual (definitive reference for LED behaviour and section 5-4 troubleshooting).
- Omron W205-E1-04 — CV/CVM1 Host Link System Operation Manual (covers CV500-LK201 and shared Host Link protocol details).
- EIA/TIA-574 — RS-232C DB9 pin assignment standard used by the PC side of the cable.
- Omron CX-Programmer Operation Manual (W446-E1) for CX-Programmer driver selection and dialog screenshots.
What does it mean when the LK201 RUN LED never lights but the CPU appears healthy?
Per Omron W143-E1-6 §5-4, the RUN LED only lights after the unit successfully negotiates with the CPU during power-up. If the CPU is healthy, the most common causes are a half-seated LK201 connector or the rear SW3 selector set to RS-422 instead of RS-232C. Reseat the unit and confirm rear SW3 = ON before suspecting a defective LK201.
Can I substitute the LK201-V1 for the LK201 without changing my CX-Programmer settings?
Yes. The -V1 uses the same DIP switch assignments and the same Host Link C-mode command set, so unit number 0, baud 9 600, 7-E-2 frame, and command level SW4 = 2 all carry over unchanged. The only practical difference is that SW3 = 6 enables 19 200 bps on the -V1, which the older CPU01 cannot match — leave SW3 = 5.
Why does RCV flash but XMT stays dark on the LK201?
The LK201 is receiving the PC's frames (RCV blinks) but is not transmitting a reply. The four most likely causes are: a swapped TX/RX pair in the cable, an incorrect unit number in CX-Programmer, an incorrect baud rate (verify SW3 = 5 for 9 600), or an incompatible command level (CX-Programmer needs SW4 = 2 for 1:1 Host Link). Confirm each in turn.
What is the maximum cable length between PC and LK201 over RS-232C?
15 m (50 ft) per the EIA/TIA-574 RS-232C standard. Above 15 m, signal integrity degrades and CX-Programmer will experience intermittent timeout errors. For longer distances, convert to RS-422 using a C200H-LK202 and twisted-pair cable.
My CX-Programmer shows the LK201 online for a few seconds, then drops. What now?
Intermittent drops are typically caused by either a noisy cabinet (use shielded cable and bond the LK201 DB25 pin 1 to chassis) or a USB-to-serial adapter adding latency. Increase CX-Programmer's response timeout from 2 s to 4–5 s, and verify the COM port driver is the FTDI or genuine Omron VCP driver, not the generic Microsoft serial driver.