CJ1M Serial Printer Output via CJ1W-SCU21-V1 Module

Overview

The Omron CJ1M CPU unit exposes three distinct external interfaces: a peripheral (USB or RS-232C "Tool"/peripheral) port, a built-in RS-232C port (PORT 1 on most CJ1M-CPU1[] / CPU2[] / CPU3[] units), and one or more Serial Communication Unit (SCU) slots populated with modules such as the CJ1W-SCU21-V1 or CJ1W-SCU31-V1. Engineers who need to print DM-area data on an Epson LX-300 (or any ESC/P-compatible 9-pin serial dot-matrix printer) frequently try to use the peripheral port with the TXD instruction first; that approach fails on the CJ1M because the peripheral port is reserved for CX-Programmer / Hand-Held Programming Console traffic and does not expose Host Link or No-Protocol transmission services.

The correct implementation requires:

1. Adding a CJ1W-SCU21-V1 Serial Communication Unit to the CJ1M rack.
2. Selecting either TXDU (No-Protocol mode, CPU unit version 3.0 or later) or PMCR with a Protocol Macro (supported on every CJ1M unit version, but requires CX-Protocol software).
3. Wiring the Epson LX-300 to the SCU's RS-232C port with a standard DB-9 straight-through or crossover cable as dictated by the printer's DTE/DCE orientation.
4. Formatting the DM-area payload so the printer receives the correct row, line-feed, carriage-return, and form-feed bytes for the LX-300 ESC/P command set.

This article documents the port-selection rationale, hardware configuration, ladder / ST programming, CX-Protocol macro authoring, and the commissioning checks needed to verify reliable serial printer output on the CJ1M platform. Reference material is drawn from the official Omron CJ1M CPU Unit Operation Manual (Cat. No. W472) and the CJ1W-SCU21-V1 Serial Communications Unit Operation Manual (Cat. No. W336).

Why the Peripheral Port Cannot Execute TXD on the CJ1M

The CJ1M peripheral port is a peripheral-bus interface intended solely for programming devices. According to the W472 manual, the peripheral port on the CJ1M CPU supports only the following services:

• Host Link slave (CX-Programmer online editing, monitoring, force set/reset)
• Programming Console bus protocol (for the CQM1H-PR001 and CS1W-PRM21 hand-held consoles)

It does not support the No-Protocol send (TXD / TXDU) mode. Any attempt to issue TXD with the destination port set to the peripheral port (port number 0 in some legacy documentation) returns error flag P_ER with a non-zero status code in A203 (communications port error). On newer CX-Programmer revisions this manifests as a "port not supported" warning during compile, but the instruction is still rejected at run-time by the CPU firmware.

Hardware Selection: CJ1W-SCU21-V1 Module

The CJ1W-SCU21-V1 is a two-port RS-232C Serial Communications Unit that occupies a single CJ-series slot. Each port can be configured independently. With one module installed, the CJ1M gains two fully programmable serial channels that can each be set to:

• Host Link master or slave (for CX-Programmer, NT-series HMIs, peer-to-peer SCU links)
• No-Protocol (asynchronous, free-format) — required for direct printer output
• Protocol Macro execution target
• Serial PLC Link (for CJ1M-CPU1[] / CPU2[] / CPU3[] peer data sharing)
• Modbus-RTU master / slave

Two module variants are commonly stocked: the SCU21-V1 (RS-232C + RS-232C) and the SCU31-V1 (RS-422A/485 + RS-232C). The Epson LX-300 ships with a 9-pin male DTE RS-232C connector on its "Serial" interface option board, so the SCU21-V1 is the correct part. If a parallel-only LX-300+II is in service, you must use the C82305* / C82306* serial-interface option kit from Epson before the SCU21-V1 will be useful.

CPU Version Determines the Programming Path

The choice of ladder instruction depends on the CJ1M CPU unit's version label, which is printed on the side sticker as "Lot No. __ _ _ _" and exposed as the first two DM words of the CPU status area (e.g. A100 / A101). The version also appears in CX-Programmer under PLC > Properties > CPU Type.

For a Version 3 CPU, the simplest path is TXDU on the SCU port. For any pre-V3 lot, you must build a Protocol Macro in CX-Protocol and call it from ladder with PMCR(260). The macro approach is more verbose but works on every CJ1M ever shipped, and it is the only viable path on early lots.

Wiring the Epson LX-300 to the CJ1W-SCU21-V1

The Epson LX-300 serial interface is a 9-pin DTE (data terminal equipment) male connector. The CJ1W-SCU21-V1 port is a 9-pin DCE (data communications equipment) female connector. For the simplest No-Protocol link with software (Xon/Xoff) flow control, a "null-modem" crossover cable is required. For hardware (RTS/CTS) flow control, a partial crossover is required.

If you choose hardware handshaking, also connect SCU pin 7 (RTS) to LX-300 pin 5 (CTS) and SCU pin 8 (CTS) to LX-300 pin 8 (RTS). Always tie signal ground (pin 5 to pin 5) — floating grounds are the single most common cause of intermittent ESC/P character corruption on the LX-300.

Configuring the SCU Port in CX-Programmer

After the CJ1W-SCU21-V1 is registered in the I/O table, open PLC > Serial Communications > Port Configuration in CX-Programmer. The dialog allocates the following setup words to the chosen port's DM area. The following parameters target the Epson LX-300 at 9600 bps, 8N1, Xon/Xoff flow control, No-Protocol mode:

The exact DM word addresses depend on the SCU's slot number and whether the CPU is configured for 1:N DM allocation. CX-Programmer writes these values automatically when you click Transfer to PLC; the values are stored in non-volatile flash on the SCU module itself, so they survive power cycle.

DM-Area Payload Layout for LX-300 Print Output

The Epson LX-300 expects printable ASCII characters terminated by carriage return (0x0D). A line-feed (0x0A) is optional if the printer's "Auto LF" DIP is set, but for predictable output it is best to include both. Page ejection is performed with form feed (0x0C). Build the print string in a contiguous DM block as follows:

In ladder, populate this block with ASCII constants and use a counter (CIO 2000.00 → CIO 2000.01 or a real CN register) converted to ASCII via ASC(286) before the TXDU call. Each DM word holds two ASCII characters in big-endian (high byte first), which is the order TXDU transmits.

TXDU Programming on a Version 3 CPU

The TXDU(256) instruction on a CJ1M-CPU12/22/13/23 V3+ SCU port has the following operand structure (full operand reference is in the W472 Instruction Reference section):

TXDU(256)
  S    : D100       ; first source word of ASCII payload
  C    : D200       ; control word block (5 words)
  N    : 12         ; number of bytes to transmit (must be even and ≤ 256)

The control block at D200 must be pre-loaded with the following:

A minimal rung that fires the print on rising edge of W0.00:

| W0.00 |---[ TXDU(256) S:=D100 C:=D200 N:=12 ]---( W1.00 )

W1.00 will turn on for one scan when the send completes successfully. On failure, examine D204 and the SCU status word A220 + n*10 (where n is the SCU unit number in the I/O table).

PMCR Protocol Macro Path (All CPU Versions)

For pre-V3 CJ1M units, or when you need to drive a printer with a more complex handshake (e.g. ESC/P status polling, multi-line form feeds triggered by buffer-full characters), build a Protocol Macro in CX-Protocol. A macro is a sequence of steps and send/receive messages stored in the SCU's flash memory. Ladder code triggers the macro with PMCR(260).

Creating the macro in CX-Protocol

1. Launch CX-Protocol and create a new project. Select Target > CJ1W-SCU21-V1.
2. Define a protocol named P001 with a sequence named S001 containing one step, S001.01.
3. Set the step's Send Message to a Variable (Word) reference pointing to D100 with length 12.
4. Set the step's Repeat Counter to 1 (no retry) and Send Wait to 0 ms.
5. Add a Receive message (optional) to clear the Xon/Xoff state after transmission.
6. Click Transfer to PLC > Protocol Data to write the macro into the SCU module's flash.

Ladder invocation

PMCR(260)
  C1 : #0000       ; control word — always 0000 for standard send
  C2 : #0001       ; protocol number (1 = P001)
  S  : #0001       ; sequence number (1 = S001)
  D  : D300        ; first result word; 4 words of status returned
  W  : W0          ; internal work word (set to 0 before call)

Full register-by-register behaviour is documented in the CX-Protocol Operation Manual (Cat. No. W344). The macro's flash footprint is small (typically < 2 KB), so dozens of print formats can coexist on one SCU module.

Multi-Device Topology: Touch Panel + Printer + Second PLC

The original application requires three serial devices to share one SCU module. The recommended assignment, consistent with the discussion from which this reference is derived, is:

Do not cascade the printer onto the same port as the touch panel. ESC/P traffic from the SCU will collide with NT-Link polling and corrupt the HMI screen update. Two independent physical ports are mandatory.

Verification and Commissioning Checks

Before declaring the print job production-ready, run the following acceptance checks in order:

1. Loopback test: Disconnect the printer, fit a DB-9 male-to-female loopback plug (TD↔RD, RTS↔CTS) on the SCU port, and run a Protocol Macro Send + Receive sequence. Verify that all 12 bytes echo back unchanged. This confirms the SCU's UART is healthy and the wiring is correct.
2. Status-word sanity: With CX-Programmer online, watch the SCU port status word (n+9 in the SCU's allocated CIO area, where n is the unit word). Bit 7 should be 0 (ready) and bit 6 should be 0 (no transmission error).
3. Print a known string: Load D100–D110 with the literal "DIAG OK v3.0\r\n". Trigger TXDU from the PLC. The LX-300 must print DIAG OK v3.0 on a single line and the green ONLINE light must remain lit.
4. Throughput test: Print 200 lines in a loop with 50 ms inter-line delay. The LX-300 must not stall, double-space, or drop characters. If characters are dropped, drop baud rate to 4800 or enable Xon/Xoff in the SCU setup word.
5. Long-run soak: Print 5,000 lines continuously. Verify that the SCU's send-complete flag does not latch on and that no 0101 (port busy) errors accumulate in D204.

Troubleshooting Matrix

Field-Engineer Checklist

• Confirm CJ1M CPU lot number is V3.0+ for TXDU shortcut, otherwise plan for a Protocol Macro build.
• Verify the Epson LX-300 has the serial interface option board installed (C82305*/C82306*) — the parallel-only unit will not respond to SCU traffic.
• Set SCU port 1 to No-Protocol, 9600, 8N1, Xon/Xoff, 12-byte send length.
• Pre-load the DM payload with the literal ASCII string, terminated by 0x0D 0x0A for each line and 0x0C for page eject.
• Trigger TXDU(256) on a single rising-edge input. Latch the send-complete flag to gate the next call.
• Run loopback test, single-line print, and 200-line throughput test before commissioning.
• Document the SCU slot number, port number, and DM block addresses on the cabinet label for future maintenance.