Bottle label barcode verification: fixed reader and PLC serial link

You are labeling bottles, and the line has to prove each one wears the right label before it packs. The dependable fix is a fixed-mount code reader over the conveyor that decodes the label's barcode and hands the ID to the PLC, which compares it with the recipe and accepts or rejects the bottle. This guide commits to one architecture - a fixed reader with a serial ASCII output wired into a spare PLC comm port - and works both sides of it: the reading distance, field of view and exposure that make the decode hold, and the baud rate, message framing and buffer sizes that make the link clean. You finish with a worked geometry, an exact serial frame, the compare logic, and a checklist you can order from.

The decision up front

Buy one fixed-mount code reader from the serial side of the catalog, aim it at the label, and let the PLC make the call. The reader outputs plain ASCII with the framing stripped to the minimum: preamble off, symbology ID off, a fixed-length label ID, a CR/LF postamble, 9600 baud, 8 data bits, no parity, 1 stop bit. Wire it TX to RX, RX to TX, signal ground to signal ground, into the PLC's second comm port, switched from its sequenced protocol to non-sequenced ASCII. The PLC accepts a bottle only when the received string equals the recipe string exactly, retries once on a no-read, and rejects anything else.

The wire is never the bottleneck. Ten characters at 8N1 is 100 bits, 10.4 ms at 9600 baud, and a 30-bottle-per-minute line gives each bottle 2 s. Serial is the wrong tool only when you run a second reader, the HMI already owns the spare port, or images must be archived; those cases close the article.

Checking the label is not grading the code

Two jobs hide inside the word verification, and they buy different hardware. Proving the right label is a string compare: decode the barcode, compare with the recipe, accept or reject the bottle. Grading print quality measures the symbol itself - contrast, defects, growth - against ISO/IEC 15416 for linear codes and ISO/IEC 15415 for 2D symbols, with a calibrated verifier; barcode reader verification for label inspection covers that discipline. A right-label station needs dependable decodes and an exact compare, not a grade. The neighboring task on bottle lines - proving the label sits straight before a servo correction - is its own subject: bottle label orientation vision for servo alignment.

Pick the interface before the focal length

Filter fixed code readers by interface first, because the interface shrinks the pool before the optics do. The Interface: Serial filter on the fixed code reader finder lists 28 models; Fast Ethernet alone lists 365. For a brick PLC whose only free input is a serial ASCII port, serial wins: one cable, one frame format, and you can watch the traffic with a terminal. The MV-ID6200PM-00C-NNG sits at the wide end of the serial pool, a 20 MP logistics code reader with a Serial interface and a C-mount lens.

Keep the RS-232 run under about 15 m at 9600 baud; past that distance, or through a noisy panel, use Ethernet and convert at the rack - a serial to Ethernet converter feeding plant data tags keeps the PLC-side ASCII intact. If your PLC has a spare Ethernet port, skip RS-232 entirely and start from planning a reliable barcode reader to PLC link.

Worked geometry: distance, field of view and pixels per module

Set the optics with one chain of arithmetic. Take a 90 mm wide label carrying an 8-character Code 128 ID, modules (the narrowest bars) of 0.25 mm, bar height 8 mm; a reader 300 mm from the bottle face; and a 1/3-inch sensor 4.8 mm wide with 1280 pixels across. These are example inputs - swap in your own label and standoff and the formulas hold.

Magnification m is sensor width over field of view, and WD is the lens-to-label working distance.

The field of view must clear the label plus placement wander, so target 100 mm. Then m = 4.8 / 100 = 0.048 and f = 300 × 0.048 / 1.048 = 13.7 mm, which rounds to the nearest stock focal length of 12 mm. At 300 mm that 12 mm lens gives m = 12 / (300 - 12) = 0.042 and a field of view of 4.8 / 0.042 = 115 mm. Resolution follows: 1280 / 115 = 11.1 px/mm, so a 0.25 mm module spans 2.8 px - above the 2 px decode floor, just under the 3 px that buys margin on marginal prints.

The code fits the label with room to skew: an 8-character Code 128 runs (11 × 9 + 13) modules × 0.25 mm = 28 mm of bars plus 5 mm of quiet zone, 33 mm inside 90 mm.

If your modules are 0.20 mm, 2.2 px reads clean labels and drops marginal ones. Halve the standoff instead: at 200 mm the same 12 mm lens covers 4.8 / (12 / 188) = 75 mm, and 1280 / 75 = 17 px/mm puts 3.4 px on a 0.20 mm module. Closer standoff buys resolution and costs mounting room and depth of field - bottle diameter spread starts to matter below about 150 mm.

Run your own label size and standoff through the same arithmetic:

The compact catalog match for this geometry:

The MV-ID3013PM-12M-WBN is a compact industrial code reader with a 12 mm focal length, 1.3 MP resolution, a global shutter and a Fast Ethernet interface - the optics of the worked example, with the interface your architecture chooses. On a slow bench line where a PC or HMI runs the compare instead of the PLC, USB saves the serial wiring:

The LS-CRD-0047 is a compact fixed industrial barcode reader with 1.2 MP, a USB 2.0 interface and a 60-120 mm reading distance - the standoff class for a reader mounted on the labeling fixture itself. Three working-distance classes from the catalog, side by side:

Values as printed in each product's data sheet; open a part for its drawing and full specifications.

Match the class to where your bracket can live; everything downstream - framing, buffer, compare - is the same logic on every row.

Trigger and exposure on a moving bottle

Motion caps the exposure, not the decoder. With 115 mm on 1280 pixels, one pixel is 115 / 1280 = 0.09 mm on the bottle. Keep blur under one pixel and the exposure ceiling is 0.09 / 200 = 450 µs at 200 mm/s of belt. Set 250 µs: blur is 200 × 0.00025 = 0.05 mm, or 0.56 px. Trigger from a photoelectric sensor upstream of the reader rather than free-running, so each frame owns one bottle and the PLC can key its verdict to the trigger bit.

Decode rate is nowhere near a limit at bottle-line speeds: 30 bottles a minute is half a read per second, and the LS-CRD-0028 is a 2560 x 2048 fixed code reader rated 60 codes/s at 30 fps with an 8 mm lens.

Gloss, curvature and frost

A glossy film label on a curved bottle is a moving mirror: the specular stripe slides across the code as the bottle turns. Use two fixes together - tilt the reader 10-15° off the label normal so the reflection leaves the lens, and buy a polarized reader. The polarized pick:

The MV-IDB005EXI-05NRP is a polarized 1.3 MP industrial code reader with a 4.63 mm focal length and a 70 mm focus distance, built for exactly this short-standoff glare problem. During teach-in, keep faces out of the illuminator beam at close range; the light is aimed at the label and that is where it stays.

Frost is not a lighting problem. Condensation on a cold-filled bottle scatters the code, and no polarizer reads through it: wipe with an air knife upstream or read after the warm section. Frosty label scanning on a fast line works this same failure at 80 units per second.

Frame the string the PLC will see

A reader's serial message is assembled from four parts, in order: an optional preamble, an optional symbology ID, the data (with an optional delay between characters), and an optional postamble. Commit to the leanest frame: preamble off, symbology ID off, your fixed 8-character ID, then CR + LF. The terminator carries the message - the PLC's ASCII receive instruction ends on it instead of counting characters into silence.

At 9600 baud with 8N1, every character is 10 bits, so 8 characters plus CR and LF is 100 bits and 10.4 ms end to end. Leave the inter-character delay at zero; if the PLC port drops characters, 1 ms per character still lands the whole frame in about 20 ms.

Turn the symbology ID back on only when two code types share the station and the logic must tell them apart - then strip those bytes in ladder before the compare. Prefix and suffix framing for barcode scanner input and barcode scanner capture without a prefix take the trimming further.

Set up the PLC port and reserve its buffer

Brick PLCs expose a spare serial port as an ASCII receiver: an AIN (ASCII In) instruction on port 2, or a Receive-style instruction on the CPU module's second port. Reading a barcode scanner into a PLC CPU module over serial covers that wiring. In the port setup - the screen is usually called Setup Secondary Comm Ports - switch the port from its sequenced protocol to Non-sequence I/O, point the Memory address at a block of 9 words the port owns, and set 9600 baud, 8 data bits, no parity, 1 stop to match the reader exactly. The port packs two ASCII characters per 16-bit word, so the 10-character frame occupies 5 words of that block.

Settings that disagree announce themselves as garbage in the buffer: fixing garbage characters from a barcode scanner on the PLC serial link points at baud, then parity, then wiring, in that order. When the same string must appear on an HMI, the byte-to-display conversion is a step of its own: barcode reader ASCII conversion and HMI display.

Compare, decide, and separate the failures

Three rungs of ladder per bottle:

1. Arm on the trigger bit and start a 300 ms watchdog.
2. On frame received, check length first - exactly 8 characters - then compare buffer words against the recipe register, two characters at a time.
3. Latch the verdict before the next trigger clears the buffer.

PLC ladder logic to compare barcode strings and count three reads gives the compare-and-count pattern, and judging whether a decode is trustworthy covers flagging marginal decodes with the reader's read-quality indicator even when the string matched.

When serial is the wrong answer

Serial ASCII is the right call for one reader and one free port. Move to Ethernet when the HMI already occupies that port, when a second reader joins the station, or when images must be kept for traceability - archiving barcode inspection images to cloud storage begins at the reader's image stream, which an RS-232 link cannot carry.

Checklist before you order

• Label ID fixed at 8 characters; X-dimension 0.25 mm or wider at a 115 mm field, 0.20 mm at 75 mm.
• Standoff and lens from the formula: 12 mm at 300 mm covers 115 mm on a 4.8 mm sensor, 2.8 px per 0.25 mm module.
• Exposure 450 µs or shorter at 200 mm/s; confirm blur stays under one pixel at your real belt speed.
• Reader frame: preamble off, symbology ID off, CR/LF postamble, 9600 baud, 8 data bits, no parity.
• PLC port in Non-sequence ASCII, 9-word block reserved, buffer read two characters per word with one guard word spare.
• Photoelectric trigger upstream, 300 ms watchdog, one retry, diverter wired to the latched verdict.
• Glare plan: polarized reader or a 10-15° tilt. Frost plan: air knife upstream of the reader.
• RS-232 cable under 15 m, TX and RX crossed, shield grounded at one end only.

Take the label artwork and your port map into the system configurator, or contact the team and we will size the station with you.