GT1055-QSBD Barcode Reader RS232 Connection Troubleshooting

Problem Overview

The Mitsubishi GT1055-QSBD is an entry-level 5.7" STN monochrome HMI from the GT10 series. It is commonly deployed as a low-cost operator panel for small machines, and field engineers frequently connect RS-232 barcode scanners to it for tracking work orders, serial numbers, and material lots. The most reported failure mode is a scanner that reads barcodes successfully on a PC (verified with a terminal emulator at 9600,8,N,1) but produces no data on the GOT: the trigger fires, the laser/CCD activates, yet nothing appears in the destination buffer or ASCII input object.

On the GT1055-QSBD the failure almost always traces to one of three causes:

1. The barcode scanner is not transmitting a terminator character (CR, LF, or CR+LF), so the GOT is permanently waiting for the end-of-message marker.
2. The ASCII input object is configured to write into a GOT-internal GD register range that overlaps an active system area or is not allocated in the project.
3. The system signals for barcode reading (GS bit 6 read-complete, GS bit 7 error) are disabled or pointing to the wrong GS word, so the PLC never sees a data-ready trigger.

Of these, the missing terminator is by far the most common field fault, and is the root cause of the original case report described in the field report.

GT1055-QSBD Hardware Reference

The GT1055-QSBD is the smallest member of the GT10 family with a Q-bus compatible communication processor. The "-QSBD" suffix decodes as: Q = Q-series compatible, S = STN monochrome LCD, B = black bezel, D = 24 VDC supply.

The single RS-232 port is shared between PLC and peripheral traffic. On the GT10 series, peripheral devices such as barcode readers are mapped to channel 8 in the GT Designer connection list — not to the PLC channel (channel 1 by default). This is the channel ID used in the field report.

RS-232 Port Pinout and Wiring

The GT1055-QSBD RS-232 port uses a male 9-pin D-sub connector. Mitsubishi's wiring for barcode readers uses the minimum 3-wire subset: TD, RD, and SG.

Wiring rule: For most handheld scanners, only pins 2, 3, and 5 are required. Connect DTR/DSR/RTS/CTS to satisfy the scanner's handshake expectation only if the scanner documentation mandates it. Most Cognex, Datalogic, Honeywell, and Zebra RS-232 scanners default to no hardware handshake, in which case pins 4-8 should remain disconnected or looped back per scanner manufacturer instructions.

GT Designer 3 Connection Setup

Step 1 — Add the barcode connection

1. Open the project in GT Designer 3.
2. In the project tree, expand Connection.
3. Right-click and choose Add New Connection or use an unused slot (typically channel 8 on GT10 series).
4. Set Connection Type to Barcode Reader (in older builds: ASCII Input).
5. Set Channel to Ch8 (RS-232 on GT1055-QSBD).
6. Set Baud rate: 9600.
7. Set Data length: 8.
8. Set Parity: None.
9. Set Stop bits: 1.
10. Set Flow control: None (unless scanner requires it).

Step 2 — Configure terminator handling

This is the single most important setting and is the cause of the original failure. In the connection's extended properties, the terminator definition tells the GOT what character marks the end of a barcode string.

The GOT default is typically CR. If the scanner is configured to send only LF (or no suffix), the GOT never sees a complete message and discards the data.

ASCII Input Object Configuration

The ASCII input object is the GOT's runtime widget that captures scanner data into the GD buffer. Place it on the destination screen, then configure its properties.

Root Cause: Missing Terminator Character

The field report was resolved when the user enabled a CR or LF suffix on the scanner. This is a near-universal pitfall because most modern RS-232 barcode scanners default to one of the following configurations:

• No terminator — the scanner only emits the barcode's raw character string. The GOT's parser waits indefinitely for an end marker; the data sits in the UART FIFO until overwritten by the next scan.
• CR terminator only — common in industrial scanners, but the GOT project may have been set to expect CR+LF, causing each scan to be discarded after the first mismatch.
• Prefix + suffix — the scanner adds an STX/ETX pair or a custom identifier. If the GOT does not strip the prefix, the data length mismatches and the buffer write is rejected.

To diagnose, connect the scanner to a PC running a serial terminal (PuTTY, Tera Term, or Hercules SETUP) at 9600,8,N,1. Trigger a scan and inspect the byte stream in hex view. The terminal must display:

• The full barcode string, followed immediately by
• The configured terminator byte (0x0D for CR, 0x0A for LF, or both)

If the terminator byte is absent, reconfigure the scanner with its programming manual or scan the manufacturer's "Enable CR/LF Suffix" configuration code.

System Signal Configuration

When the GOT is connected to a MELSEC PLC and the application needs to react to a scan event, use the GOT's system signal area (GS devices) so the PLC receives a hardware-style handshake.

Configure the system signal area under Common → System Signal → Use System Signal. Set the start device to a free GS range, for example GS0. Verify each bit in the GT Designer [System Signal] dialog before downloading.

PLC ladder excerpt (MELSEC iQ-R / Q series)

// Read-complete handshake
[ GS0.b6 ]----[ RST  M100 ]   // Acknowledge: clear M100 one-shot
[ GS0.b6 ]----[ SET  M101 ]   // Set PLC-side "data available" flag
[ M101    ]----[ MOV  GD1000 D200 ]  // Copy scanned string to D200
[ M101    ]----[ MOV  GD1001 D201 ]
[ M101    ]----[ MOV  GD1002 D202 ]
[ M101    ]----[ MOV  GD1003 D203 ]
[ M101    ]----[ RST  M101 ]   // Self-reset after processing

Each GD word stores one ASCII character in its low byte (big-endian for ASCII). Use MOV to map consecutive GD words into a D-register block on the PLC.

Barcode Scanner Configuration Reference

The following are typical parameter codes used by industrial RS-232 scanners (Cognex DataMan, Datalogic QuickScan, Honeywell Voyager, Zebra DS2208). Verify against your specific scanner's programming manual.

Verification Procedure

After configuration changes, verify the link using the following checks before declaring the connection operational:

1. Local loopback test — Loopback pins 2-3 on the GT1055 RS-232 port and write data from GT Designer self-test to confirm the UART is functional.
2. PC sniff — Disconnect the scanner from the GOT and connect it to a PC. Trigger a scan and confirm in a terminal that the terminator byte is present.
3. GOT monitor — In GT Designer 3 runtime, open Diagnostics → Buffer Memory Monitor. Watch GD1000 through GD1023 while scanning. Each scan should populate the GD range with the barcode ASCII string followed by a zero-padded terminator.
4. GS bit monitor — Monitor GS0.b6 in the buffer monitor. It must pulse ON for one scan cycle after each successful read.
5. PLC transfer test — Trigger the PLC to copy GD to D and confirm the data appears in the PLC's data register block.
6. Long-duration stability — Run 500+ scans over 30 minutes. Check for buffer overrun (GS0.b7 rising), which would indicate scanner sends data faster than the GOT processes.

Troubleshooting Matrix

Advanced Configuration: Direct Read Mode vs Triggered Mode

The ASCII input object offers two operational modes that change the handshake behavior:

Direct Read Mode (Read directly into object = Yes):

• Data flows from the scanner directly into the ASCII input object's display.
• No GS bit is required.
• Simpler; ideal for "scan then enter" UI patterns.
• Disable Use system signal area.

Triggered Mode (Read directly into object = No):

• The PLC sets GS0.b10 to start a read operation.
• GOT populates the GD buffer and sets GS0.b6 when complete.
• PLC must reset GS0.b6 by pulsing GS0.b12.
• Enable Use system signal area and allocate the system signal GS range in Common → GOT Setup → System Signal.

For production lines that require deterministic read windows, prefer Triggered Mode. For scan-and-confirm UIs, Direct Read Mode is sufficient.

GT1055-QSBD Limitations and Workarounds

The GT1055 is a budget-targeted GOT, so some features available on GT15/GT16/GT27 panels are absent:

• No dedicated Barcode/RFID object: Use ASCII Input only.
• One RS-232 channel only: Cannot use channel 8 for barcode if channel 1 is occupied by PLC traffic on the same physical port — verify that channel 8 is mapped to the GT1055 RS-232 port in the project, not a phantom channel.
• No Ethernet on the basic model: GT1055-QSBD is RS-232 only. If Ethernet barcode readers are needed, migrate to GT1055-QSBD with an Ethernet option (where available) or upgrade to GT2107-WTBD / GT2104-RTBD.
• Limited GD capacity: 65 535 words is normally sufficient, but applications that log 50+ scans in a recipe buffer must use the PLC side for long-term storage.

Related Mitsubishi Documentation

For deeper reference, consult the following Mitsubishi Electric technical documents:

• GOT-A-0133-C (PDF) — Technical bulletin on barcode reader and serial printer connection to GOT series.
• GT1055 User's Manual (SH-080525) — Hardware installation, wiring, and specifications.
• GT Designer 3 Version1 Operating Manual (SH-080859) — Screen creation, object configuration, and communication setup.
• GOT1000 Series Connection Manual (SH-080032) — Channel mapping and protocol details.

Frequently Asked Questions