Configuring Omron CPM2A Host Link for Remote Modem Access

Overview

The Omron CPM2A is a compact brick-style PLC equipped with a single RS-232C peripheral port and, on most CPU variants, an RS-232C or RS-422A option board slot. For remote engineering access across the PSTN (Public Switched Telephone Network), the CPM2A's Host Link (SYSWAY) serial protocol can be carried transparently through a pair of dial-up modems. This allows an engineer in one state or country to dial into a PLC in another location and use CX-Programmer to upload/download the program, monitor I/O, force bits, and edit data tables online.

Host Link is Omron's legacy master/slave SCADA protocol documented in the CPM1/CPM1A/CPM2A/CPM2C Serial Communications Manual. Every CPM2A ships with Host Link support built into the peripheral port; no special option board is required for the standard 7-bit even parity 2-stop framing mode at 9600 bps that this article describes.

Prerequisites

Before configuring the link, verify that the following hardware and software are present.

Avoid internal (PCI/USB bus) modems on the engineering PC. They share IRQ resources with CX-Programmer's COM port and frequently produce framing errors during the long-lived Host Link sessions used for online monitoring.

CPM2A Peripheral Port Configuration

The CPM2A peripheral port is fixed at Host Link (SYSWAY) slave mode on power-up — it cannot be re-purposed without writing DM parameters into the PLC. To confirm or change the serial framing, the DIP switch on the front of the CPU must be set, and the relevant DM words must contain the default values listed in the table.

The peripheral port's fixed communication parameters are:

• Baud rate: 9 600 bps (cannot be changed on the peripheral port)
• Data bits: 7
• Parity: Even
• Stop bits: 2
• Start bits: 1
• Frame: 11 bits total (1-7-E-2)
• Protocol: Host Link (SYSWAY), 1:1 (one-to-one) mode — the host (PC) is unit #00, the PLC is the value held in DM6651

RS-232C Cable Pinout (PLC Side)

The cable between the CPM2A peripheral port (DB9 male on the CPM2A-CN111 adapter) and the modem (DB25 male on a typical industrial modem, or DB9 female on a desktop unit) must perform a null-modem crossover. The pinout below matches the standard Omron programming cable so the same harness works for both serial programming and modem pass-through.

Pin 1 (CD) and Pin 9 (RI) are not used for Host Link. If the modem does not assert DSR, jumper pins 6-1-4 together on the PLC end so that the Host Link driver sees an "always ready" state. Many CPM2A-CN111 cables do this internally; verify with a continuity tester if Carrier Detect never rises in CX-Programmer.

Modem Configuration — Remote Site (PLC End)

1. Connect the modem's serial port to the CPM2A peripheral cable as wired above.
2. Connect the modem's line jack to the analog PSTN extension.
3. Connect an analog handset to the modem's PHONE jack — this is critical for the operator-dial workflow described below.
4. From a terminal program on the PLC side (or by writing the modem's NVRAM), store the following AT command string:

   AT&F1&W0
   ATS0=1E0Q1&W0
   AT&K3\N3&W0

5. The first line resets the modem to a known factory profile ("&F1" on USR/Multi-Tech). The second line sets auto-answer on the first ring (S0=1), disables command echo (E0), and enables result codes (Q1) so the Host Link driver sees clean CONNECT/CARRIER responses. The third line enables hardware RTS/CTS flow control (&K3) and normal-speed buffering (\N3) and stores the profile to NVRAM (&W0).
6. Cycle power to the modem and re-dial from a handset to confirm the modem answers, hisses, and connects.

CX-Programmer Configuration — Engineering PC

1. Launch CX-Programmer. From the menu bar select PLC > Edit Network Configuration. When prompted, choose New and select the network type Sysmac WAY. Do not select SYSMAC NET, SYSMAC LINK, or Ethernet — those are routed network types that require an intermediate FINS gateway.
2. Right-click the new network entry and choose Modify. In the dialog box set:
   • Network Address: 0
   • Node Address (Local): the unit number configured on your PC's Host Link driver (use 00 unless you are bridging through another PLC)
   • Network Address (Remote): same — Sysmac WAY is a single-segment serial network
3. Select the Modem tab. Click Add, then choose the installed modem from the device list. If the device list is empty, install the modem's INF driver from the manufacturer disk first.
4. Click Configure for the modem entry. In the configure dialog match the modem's serial settings to the CPM2A: 9 600 bps, 7 data bits, Even parity, 2 stop bits. Set flow control to Hardware (RTS/CTS). Save.
5. In the Modem tab, tick Use Operator to Dial. With this option enabled CX-Programmer waits for the engineer to dial manually, hear the modem handshake, hang up the phone, and click OK before continuing the connect sequence. This is invaluable for diagnosing line-quality issues in the field.
6. Fill in the Country, Area Code, and Phone Number fields — even if Use Operator to Dial is enabled. Leaving these blank triggers the legacy CX-Programmer bug "Selected port is in use by another application" on the Work Online dialog, and the bug is not fixed in CX-Programmer v5.0. Enter the actual digits of the remote site's line so CX-Programmer has a string to validate, then ignore the auto-dial string.
7. Click Work Online from the PLC menu. The dial dialog appears; lift the handset, dial the remote site's number, listen for the answering modem's whistle, wait until the PLC modem answers and switches to data mode (you will hear the line go quiet), hang up the handset, then click OK in CX-Programmer.
8. CX-Programmer transmits the Host Link unit-number handshake to the CPM2A and, on success, transitions to Online mode. The status bar displays the unit number and the current cycle time of the CPM2A.

Verifying the Host Link Session

After clicking OK in the Work Online dialog, perform these checks to confirm a healthy link.

If the link drops after a few minutes, suspect modem buffer overrun. Lower the connection rate to 4800 bps by writing DM6650 = 0000 with the cycle-time scan time of the CPM2A lengthened (DM0009) — Host Link uses window-based flow control and a slow PLC scan can starve the modem buffer. Alternatively, replace the modem with a "reliable-mode" industrial unit such as the Multi-Tech MT2834ZPX or the Phoenix Contact PSI-MODEM/ETH for sites where line quality is marginal.

Troubleshooting Matrix

Operational Notes for Field Engineers

• The Host Link frame is documented in the CPM1/CPM1A/CPM2A/CPM2C Serial Communications Manual (W356-E1) — keep a printed copy at the site for offline reference.
• If the remote site has a CP1L-EM or CP1H-EM available, prefer the Ethernet-based FINS/TCP route. The CJ-series Communication Units family and NX-series Communications Interface Units describe modern equivalents that bypass the PSTN entirely.
• For multi-vendor environments, review the Omron Communication Protocols guide — Toolbus, FINS/UDP, FINS/TCP, and EtherNet/IP each serve different use cases on the NJ/NX and CS1 platforms.
• Always power the modem from an uninterruptible source. A momentary PSTN loss drops the Host Link session and may leave AR0804 latched in the CPM2A — clear it with the CX-Programmer "Reset All Errors" function.
• Document the modem's NVRAM profile on a sticker affixed to the modem chassis. Field service personnel arriving without the AT command string waste hours re-commissioning the link.

FAQ