NT20S to C200HS-CPU01 Host Link Communication Troubleshooting
The Omron NT20S-ST121B-EV3 Programmable Terminal (PT) is a common field-installed HMI for legacy SYSMAC C-series racks, but pairing it with a C200HS-CPU01-E (or CPU01) introduces several Host Link constraints that do not exist on newer CPU21-E or CP1E platforms. This reference documents the full troubleshooting path for an NT20S that powers up, accepts an NTST project, and displays "connecting to host" indefinitely while the C200HS shows no comm activity on the peripheral port or limited activity on the LK201 unit. Each failure mode is mapped to the specific DIP-switch, cable, or DM area parameter that resolves it.
1. Affected Hardware and Firmware
| Item | Catalog Number | Notes |
|---|---|---|
| PLC CPU | C200HS-CPU01-E | SYSMAC C200HS, RS-232C peripheral port, no built-in Host Link port |
| PLC CPU (alt) | C200HS-CPU21-E | Same form factor, supports larger DM area and 15K program |
| HMI Terminal | NT20S-ST121B-EV3 | Monochrome STN, RS-232C + RS-422 host port, EV3 hardware revision |
| Host Link Unit | C200H-LK201 | Single-port RS-232C/422 Host Link module, occupies one slot of the C200HS backplane |
| Peripheral Adapter | CPM1-CIF01 | Converts peripheral port to DB9 RS-232C; works on C200HS, not on legacy C200H |
| Programming Cable | CBL-202 | DB9-to-DIN peripheral port programming cable |
| PT-to-LK201 Cable | CBL-803 / CBL-804 / XW2Z-200S | DB9-to-DB25 or DB9-to-DB9 Host Link cable set |
| PT Tool Software | NTST V4.2 / V4.6 | NT Support Tool, run on Windows 9x/NT 4.0, transfers at 9.6 kbps or 38.4 kbps |
Reference: C200HS Programmable Controllers Operation Manual (W235-E1-05); C200H/C200HS System Configuration Manual.
2. Host Link Architecture for the C200HS
The C200HS-CPU01-E has exactly one RS-232C channel: the peripheral port on the CPU faceplate. This port is wired as a programming console interface, not as a generic Host Link slave. Two viable paths exist for connecting an NT20S:
- Peripheral-port path: Attach a CPM1-CIF01 adapter, then use a CBL-202-style cable to the NT20S host port. This works on the C200HS but not on the older C200H. The CPU treats the connection as a Sysway/Host Link session, and the NT must be configured for unit number 00 with the correct port mode.
- Host Link Unit path (recommended): Install a C200H-LK201 in any slot of the C200HS backplane. The LK201 presents a true Host Link slave port and supports DIP-switched baud rates, node addresses, and CTS/RTS handshaking. Use either a CBL-803 (DB9 to DB25) or a CBL-804 (DB9 to DB9) plus a 9-to-25-pin adapter.
3. Cable Selection and Pinout
Omron's NT-series PTs default to Host Link with RS-232C on the "ONE" port (DB9 male on the NT20S). The C200H-LK201 exposes a DB25 female port. The factory Host Link cable for an NT2xS to a DB25 Host Link unit is the CBL-803; the CBL-804 is the DB9-to-DB9 variant used when the LK201 is the only DB9 device on the segment. The XW2Z-200S is the equivalent part stocked in Canada.
| NT20S DB9 Pin | Signal | Direction | LK201 DB25 Pin | Function |
|---|---|---|---|---|
| 2 | RD (RXD) | Out of PT | 3 | Receive data into LK201 |
| 3 | SD (TXD) | Into PT | 2 | Transmit data from LK201 |
| 4 | DR (DSR) | Into PT | 6 + 20 | Data Set Ready + DTR tied high |
| 5 | SG (GND) | — | 7 | Signal ground |
| 6 | ER (DTR) | Out of PT | 20 | Data Terminal Ready |
| 7 | RS (RTS) | Out of PT | 5 | Request To Send |
| 8 | CS (CTS) | Into PT | 4 | Clear To Send |
4. LK201 DIP Switch Settings
The C200H-LK201 has a 4-pole DIP switch on the front face. The standard switch positions for 9600 bps, 7 data bits, even parity, 2 stop bits, Host Link slave, unit number 00 are:
| Switch | Function | Position | Meaning |
|---|---|---|---|
| SW1-1 | Node number MSB | OFF | — |
| SW1-2 | Node number bit 1 | OFF | — |
| SW1-3 | Node number bit 2 | OFF | — |
| SW1-4 | Node number LSB | OFF | Node 00 |
| SW2-1 | Run / Stop on comm error | OFF | Continue run |
| SW2-2 | RS-232C / RS-422 select | OFF | RS-232C with CTS/RTS |
| SW2-3 | Baud rate bit 1 | ON | — |
| SW2-4 | Baud rate bit 2 | OFF | — |
| SW3-1 | Parity | OFF | Even |
| SW3-2 | Stop bits | ON | 2 stop bits |
| SW3-3 | Data bits | ON | 7 data bits |
| SW3-4 | Baud rate LSB | ON | 9600 bps total |
The reported working combination of 0-0-5-2 (where 0 = OFF and 1 = ON) is the documented factory default for 9600, 7, E, 2 on the LK201. If the switches do not match this map exactly, the LK201 will still respond electrically (LEDs flash) but the NT20S will time out waiting for a valid Host Link header. Cycle power to the LK201 after any DIP change so the new settings take effect.
5. NT20S PT Configuration in NTST
The NT Support Tool V4.2 / V4.6 project must be configured for Host Link (SYSWAY) on the port the cable occupies. Within the PT Configuration dialog in NTST:
- Select Comm. Port A or Comm. Port B depending on which physical DB9 the cable uses (Port A is the ONE port on the bottom of the NT20S).
- Set Comm. Method = Host Link (SYSWAY).
- Set Baud Rate = 9600.
- Set Data Bits / Parity / Stop = 7 / Even / 2.
- Set Unit Number = 00 (must match the LK201's node number; the CPU peripheral port is always node 00 from the NT's perspective).
- Set the PT Control Area to a valid DM word — for example DM0100. The first word is the screen number; the second word is the page/tab control word.
- Disable Host Link on the other port by setting it to None. The NT20S allows Host Link on only one port at a time; leaving the second port in Host Link mode silently disables the first.
6. PLC DM and Control Area Mapping
The C200HS-CPU01-E has a DM area from DM0000 to DM0999. The NT20S reads two consecutive words from the configured PT Control Area starting address:
| Word Offset | Function | Direction | Example Value |
|---|---|---|---|
| n + 0 | Screen number to display (1 to 2000 depending on project) | PLC -> PT | 0001 |
| n + 1 | Page / tab switch / pop-up control | PLC -> PT | 0000 |
| n + 2..n+m | PT status (backlight, buzzer, alarm history clear, etc.) | PT -> PLC | — |
For the standard DM0100 setup used in the source case, write MOV(021) #0001 DM0100 in the PLC ladder (CX-Programmer) to force the NT to display screen 1. If DM0100 reads back 0000 or contains a non-decimal value such as hex 000A, the NT silently ignores it and shows "connecting to host".
7. Common Failure Modes
| Symptom | Root Cause | Resolution |
|---|---|---|
| NT shows "connecting to host" indefinitely, no LK201 LED activity | Wrong cable or no comm on peripheral port | Use CBL-803 or CBL-804 + 9-to-25 adapter into LK201 DB25; do not use CBL-202 into the peripheral port for HMI |
| LK201 RXD/TXD flash, NT still shows "connecting to host" | Baud/parity mismatch or RS-422 mode active on LK201 | Set LK201 SW2-2 OFF (RS-232C); verify 0-0-5-2 switch map; match NT to 9600,7,E,2 |
| LK201 LEDs flash, NT receives a screen, then screen goes blank | PT Control Area word 0 = 0 or out of project range | Pre-load a valid screen number (1..N) in the first DM word; ensure project was compiled with that screen |
| Comm works on C200HS-CPU21-E but not on C200HS-CPU01-E | CPU01 DM area boundary; PT Control Area placed past supported range | Keep PT Control Area inside DM0000..DM0999; do not overlap with DM1000..DM1999 expansion that only the CPU21 supports |
| CX-Programmer can download via peripheral port, NT still no comm | Peripheral port is programming-only; not a Host Link port | Add C200H-LK201 module to the rack and use Host Link path; CX-Programmer comms prove only that the port works electrically, not as Host Link |
| NT works at 9.6 kbps but fails at 38.4 kbps | LK201 does not support 38.4 kbps in all firmware revisions | Force NT to 9.6 kbps in NTST; only the LK201-PV1 supports higher rates |
8. Diagnostic Procedure
Execute the following checks in order. Each step is independent and uses only the existing hardware — no oscilloscope required.
- Verify CPU model. Read the label on the CPU. The string "C200HS-CPU01-E" must include the "S".
- Verify LK201 presence and seating. Power off the rack, remove the LK201, reseat it firmly. A half-seated module will still power its LEDs but will not handshake.
- Read LK201 DIP switches. Set SW1 to all OFF (node 00), SW2 to OFF-OFF-ON-OFF (RS-232C, 9600 bps), SW3 to OFF-ON-ON-ON (E, 2 stop, 7 data, 9600 bps). Power-cycle the rack.
- Confirm cable pinout. Use a multimeter to verify continuity on the table in Section 3. Pins 4 and 5 on the NT20S DB9 should NOT be looped back — the LK201 provides CTS/RTS.
- Check the PT Control Area. In CX-Programmer, online monitor DM0100. Force the value 1. The NT should change from "connecting to host" to screen 1 within 1 second.
- Validate NTST project transfer. Re-send the project to the NT20S at 9.6 kbps. The NTST download itself uses the same port; if NTST downloads successfully, the cable and port mode are confirmed correct.
- Confirm single Host Link port. In the NTST PT Configuration, set the unused port to None. Save and re-send.
- Check for duplicate node numbers. If a CPM1-CIF01 is still plugged into the peripheral port and configured for Host Link, the NT will see two unit-00 slaves and fail arbitration. Unplug the CIF01.
9. Step-by-Step Resolution (Recommended Path)
The following sequence resolves the source-described problem: NT20S shows "connecting to host" while the LK201 RXD/TXD flash and the C200HS peripheral port shows no activity.
- Power down the C200HS rack.
- Confirm the CPU nameplate reads "C200HS-CPU01-E" (not "C200H-CPU01-E").
- Install a C200H-LK201 in an empty slot of the CPU rack. The LK201 is a single-width module and draws 250 mA at 5 VDC from the backplane.
- Set the LK201 DIP switches to OFF-OFF-OFF-OFF / OFF-OFF-ON-OFF / OFF-ON-ON-ON (node 00, RS-232C, 9600/7/E/2).
- Connect a CBL-803 (DB9-to-DB25) from the NT20S Port A to the LK201 DB25. If only a CBL-804 is available, add a 9-to-25-pin gender adapter. Verify RS-CS handshaking per the pinout table.
- Open the NTST V4.6 project for the NT20S. In PT Configuration, set Port A to Host Link / 9600 / 7-E-2 / unit 00, Port B to None. Set PT Control Area to DM0100.
- Transfer the project to the NT20S at 9.6 kbps.
- In CX-Programmer, online edit the C200HS. Add a first-scan or always-on rung:
MOV(021) #0001 DM0100. This forces the NT to display screen 1 on power-up. - Power cycle the C200HS, then power the NT20S.
- Verify: the NT leaves "connecting to host" within 1 second and displays screen 1. The LK201 SD/RD LEDs flash at 1 to 5 Hz during steady-state polling.
10. Verification Checklist
| Check | Pass Criterion |
|---|---|
| NT20S leaves "connecting to host" within 2 s of power-up | Screen 1 displayed |
| LK201 SD and RD LEDs blink at a steady rate | 1 to 5 Hz with 10 screens polled |
| CX-Programmer online monitor shows DM0100 changing when a screen-button is pressed | Yes, value updates within poll interval |
| Writing DM0100 = 5 from CX-Programmer forces NT to screen 5 | Screen change within 1 s |
| No PLC COMM error indicator on the C200HS CPU faceplate | No flashing red LED |
| Disconnect cable: NT shows "connecting to host" within 5 s | Yes, recovery to "connected" within 2 s of reconnection |
11. Field-Proven Caveats
- CX-Programmer online edits over the peripheral port do not interfere with the LK201 path, but if the CIF01 is left connected and set to Host Link, the NT will intermittently see the CIF01 as a unit-00 collision. Unplug the CIF01 after programming.
- DM area boundary: The C200HS-CPU01-E supports DM0000..DM0999. Placing the PT Control Area at DM1000 will compile but never update. The C200HS-CPU21-E extends to DM1999; this is the only operational difference that matters for an NT20S application, and it does not by itself explain a "connecting to host" symptom.
- Baud-rate trap: The NTST project default is 19.2 kbps. The LK201 at 0-0-5-2 is 9.6 kbps. If the project is sent at one rate and the runtime port is set to the other, the NT will display project screens but will not poll the PLC.
- Hand-held programmer port behavior: The C200HS peripheral port is shared between the programming console protocol and Host Link. Some firmware revisions revert to programming-console mode on power-up; the LK201 path avoids this ambiguity.
- EV3 hardware revision: The NT20S-ST121B-EV3 uses the same Host Link frame format as the earlier EV1/EV2 revisions. No NTST project recompile is required when moving a project from a legacy EV2 unit to an EV3.
12. Frequently Asked Questions
Can the NT20S communicate with a C200HS-CPU01-E on the peripheral port?
Yes, but only by attaching a CPM1-CIF01 adapter and using Host Link on the peripheral port. The peripheral port on the C200HS is electrically a programming-console port, not a guaranteed Host Link slave. The recommended path is a C200H-LK201 in the backplane; this avoids the peripheral-port ambiguity and works on the older C200H as well.
What DIP switch setting on the C200H-LK201 gives 9600, 7, E, 2?
SW1 all OFF (node 00), SW2 OFF-OFF-ON-OFF, SW3 OFF-ON-ON-ON. This is the documented 0-0-5-2 combination: SW1=0, SW2=0, SW3=5 (binary 0101 = ON-ON-ON-OFF read low-to-high, but in the LK201 manual's 1-to-4 ordering it is OFF-ON-ON-ON), SW4=2 (no fourth switch on the LK201; the "2" refers to the 2 stop bits selected by SW3-2).
Why does the NT20S show "connecting to host" when the LK201 LEDs are flashing?
Most often because the LK201 is in RS-422 mode (SW2-2 ON) while the cable is wired for RS-232C, or because the NTST project sets a different baud rate than the LK201. Verify SW2-2 is OFF and the NT project is set to 9600, 7, E, 2. A second common cause is the PT Control Area first word being 0 or out of range; the NT then has no screen to call and remains in the connect-attempt state.
Is a CBL-804 with a 9-to-25 adapter equivalent to a CBL-803?
For the C200HS-LK201 path, yes — both produce the same logical pinout on the LK201 DB25. CBL-803 is the factory-preferred cable; CBL-804 plus adapter is the field-workable alternative. Always meter the continuity before applying power, because some 9-to-25 adapters cross pins 2/3 and invert TX/RX.
Does the C200HS-CPU01-E support a PT Control Area above DM1000?
No. The CPU01 supports DM0000..DM0999 only. The CPU21 extends the area to DM1999. If your NTST project specifies a PT Control Area of DM1000 or higher, the NT will not display any screen and will appear stuck on "connecting to host". Change the PT Control Area to DM0100 or lower.