Centralized logic
The PLC runs zone logic for every zone through networked cards or I/O. One program to read, one place to change behavior.
Conveyor PLC programming
Conveyor PLC programming for merges, diverts, and tracking
Logic for MDR and roller conveyor systems written against a signed functional design: zone supervision, carton tracking, merges, sortation, WMS/WCS interfaces, and HMI diagnostics. Allen-Bradley Logix and Siemens TIA Portal.
Architecture
Central, distributed, or both
The right split depends on the size of the system, the MDR platform, and who will support it after startup. We recommend one and write it into the functional design.
Distributed with supervision
MDR cards run accumulation locally while the PLC handles modes, faults, and release commands. Fewer points in the PLC, more configuration to document.
Mixed
Cards run plain accumulation. The PLC takes direct control at merges, diverts, scanners, and anywhere carton identity matters.
Reusable blocks
Each zone, section, merge, and divert uses the same block and data structure: AOIs and UDTs in Logix, FBs and UDTs in TIA Portal.
State machines
Sections run through defined states such as stopped, starting, running, accumulating, jammed, and faulted, so behavior is predictable and easy to troubleshoot.
Modes
Auto, manual, and maintenance modes defined per section, with what each mode allows written down before code is written.
Tracking
Knowing which carton is where
Once a carton is scanned, its record has to follow it zone by zone until it reaches its destination. Most mis-sorts come from tracking that loses step after a jam or a manual intervention.
FIFO by zone
Carton records move with the carton, zone to zone, confirmed by photo eyes at each handoff so data and product stay matched.
Checkpoints
Rescans or presence checks before a sorter confirm the record still matches. A mismatch goes to exception handling, not to a guess.
Jam recovery
Records for a jammed section are held while it is cleared. Cartons removed or moved by hand are flagged for rescan.
Merges & sortation
Merges, diverts, and sorters
Rate is usually lost at merges and diverts. Logic there gets the most design time and the most testing.
Merge priority
Fixed priority, alternating, or by queue depth, chosen to keep the discharge full without starving a lane for long.
Gap control
Gaps set for what comes next: a clean read at the scanner and enough room for the sorter to divert without hitting the next carton.
Divert confirm
A sensor confirms every divert. A missed divert is logged and the carton is treated as an exception so counts stay right.
Lane full
Lane-full and line-full eyes stop feeding a lane before it backs into the sorter. Full lanes reroute or recirculate as the design says.
Exceptions
No-reads, no destination, overweight, or out-of-dimension cartons go to a reject or recirculation lane, never the wrong door.
Rate checks
Merge and sorter rates calculated from product length, gap, and speed during design, then verified with real cartons on site.
Interfaces
WMS and WCS handshakes
Routing decisions stay in the business system. Motion and timing stay in the PLC. The interface between them is written down field by field.
Message set
Scan, route request, route reply, divert confirm, and lane status messages defined with fields, lengths, and examples.
Timeouts & defaults
What the conveyor does if a route reply is late or never comes, decided in the design instead of at 2 a.m.
Heartbeat
A heartbeat both ways, so a lost connection is an alarm and a defined behavior, not a silent stop.
Testing
Interface tested against a simulator or your test system before site, then end to end with your IT team during commissioning.
Diagnostics
HMI screens a technician actually uses
Zone map
A layout-based screen showing every zone as occupied, clear, jammed, or faulted, so techs walk straight to the problem.
Alarm text
Alarms name the zone, the device, and the likely cause: a blocked eye, a card fault, a tripped branch, or a lost card on the network.
Counts & rates
Cartons per hour by section and sorter lane, plus jam counts by zone, so recurring trouble spots stand out.
Before site
Tested against simulated zones first
Emulation
Logic runs against simulated zones and cartons before installation, so most sequence errors are fixed off site.
Test plan
A written test for each merge, divert, exception path, and safety function, used at FAT and again on site.
Readable code
Commented logic, one tag standard, and no tricks. Your team or another integrator can support it later.
Start with a functional design
Every programming job starts with a written sequence of operation that you review and sign. It is cheaper to change a paragraph than a program. Our sister site has a walkthrough of what a functional design document covers.
Next step
Have a conveyor programming scope?
Send the layout, the existing program if there is one, and how routing decisions are made today. An engineer will reply with questions and a next step.
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Request received
Thanks. An engineer will follow up within one business day. Anything to add? Email [email protected].