Symptom
The download command is greyed out, the software reports that it cannot connect or that the controller is the wrong type or revision, the download begins and stops with an error, or it completes and the processor faults immediately.
The short answer
Program Will Not Download
A download fails for one of a handful of reasons: the software cannot reach the processor, the processor is in run mode with the key preventing a download, the project was built for a different processor type or firmware revision, the program is larger than the memory, the processor is protected, or the connection drops partway through. The error message names the category more often than people give it credit for. Read it, check the path and the mode, then the revisions, then the memory.
Key points
- A download is refused in run mode unless the key or the mode permits it; the mode is the first thing to check.
- The project targets a processor type and a firmware revision, and both must match the hardware in the chassis.
- The communication path has to reach the processor: right driver, right address, right slot, on a network the laptop can actually see.
- A processor that faults immediately after a download is usually an I/O configuration that does not match the chassis.
- Downloading replaces what is running. Upload first if there is any doubt about which copy is current.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Processor in run mode with the key in run | The download option is unavailable or refused. Turn the key to remote or program, or set program mode from the software if the key is in remote. |
| No communication path | The software cannot connect at all. Check the cable, the link light, the laptop address on the right subnet, the driver, and the slot number in the path. |
| Wrong processor type in the project | The error names a type mismatch. The project was created for a different controller. Change the controller type in the project properties, and review what that changes. |
| Firmware revision mismatch | The error names a revision. The processor firmware is older or newer than the project. Flash the processor to the project revision, or change the project revision, according to what the site standard says. |
| Program too large for memory | The download starts and fails with a memory error, or the software warns before starting. Reduce the program, or the data table, or use a processor with more memory. |
| Processor protected | A password, a source protection scheme, or a security authority that the laptop is not part of. The error says so; the owner of the credentials is the fix. |
| Connection drops during the download | A poor Ethernet link, a wireless connection, or a serial cable with a marginal connector. Use a wired link directly to the processor for downloads. |
| Download succeeds and the processor faults | The I/O configuration in the project does not match the chassis. Compare the configured modules and revisions against what is installed. |
Before you download at all
Work through it in order
- 1
Can the software see the processor?
Browse the network from the software. If the processor does not appear, the problem is the path: cable, link light, the laptop address and subnet, the driver configuration, or a firewall on the laptop. A processor you cannot browse to cannot be downloaded to.
- 2
What mode is it in?
Run with the key in run refuses downloads on most platforms. Remote lets the software change the mode. Program accepts a download. The key position at the panel decides which of these is available from the laptop.
- 3
Does the project match the hardware?
Open the project properties and compare the controller type and the firmware revision against what is written on the processor and shown when browsing. A mismatch in either is refused, and the message says which.
- 4
Is there room?
The software reports the program and data sizes and the processor reports its capacity. A program at the limit fails partway through.
- 5
Is it protected?
Passwords, source protection, and security authorities all refuse downloads from a laptop that lacks the credentials. The message names it. Do not try to work around it; find the credentials.
- 6
Is the link stable?
A download over a wireless bridge or a marginal serial cable fails partway. Plug in directly to the processor for the download.
Revisions
The project file, the programming software, and the processor firmware each carry a major revision, and the three have to agree. The usual way this goes wrong is a replacement processor from stock carrying a different firmware than the one it replaced, or a laptop with a newer software version that has silently upgraded the project. The site standard should say which revision the site runs on and the rack drawing should record it, so that the choice between flashing the processor and changing the project is made deliberately rather than at two in the morning.
The download that works and then faults
A download that completes and leaves the processor faulted with an I/O error is almost always a configuration mismatch: a module in the project that is not in the chassis, a module in the chassis of a different part number than the project expects, or a remote connection to something that is not there. Compare the I/O tree in the project against the physical chassis slot by slot. On platforms that check module revisions, a compatible newer module can still be refused by a configuration set to exact match.
Afterward
Verify the processor is in run, the I/O light is solid, and the process has resumed. Save the project with a note of what changed and where the copy lives. A download is a change to the plant, and the next person needs to know it happened.
Frequently asked questions
- Why is the download option greyed out?
- The software is not connected to a processor that will accept one. Either there is no connection, or the processor is in run with the key in run. Connect, then change the mode.
- The software says the controller is the wrong revision. Should I flash it?
- Only if the site standard says that revision is the target. Flashing changes the processor firmware for everything that runs on it. The alternative is to change the project revision to match the processor, which may be the right answer if the processor is the one that was already running the plant.
- Can I download while the plant is running?
- A full download requires program mode, which stops logic and drives outputs to their configured state for the duration. Online edits change logic while running and are the tool for small changes. A full download is planned with the process in a state that can tolerate the stop.
- Should I upload before I download?
- Yes, whenever there is any doubt that the saved project is current. Online edits made since the last save exist only in the processor, and a download erases them.
Related topics
- The PLC ProcessorWhat the processor module does on every scan, how its memory is organized, what the mode switch and status lights mean, and how it faults and recovers.
- Processor FaultedA PLC with its fault light on and no logic running. How to read what it is telling you, the causes in order of likelihood, and how to restart it without repeating the fault.
- Program OrganizationStructuring a controller program so that someone else can maintain it: a layout that follows the plant, one routine per job, standard device blocks, I/O mapped in one place, a fixed execution order, naming that means something, and comments that say why.
- Retentive Data LostThe plant came back from a power cycle with zero setpoints, zero totals, and the wrong pump as lead. Which backup failed, which restore overwrote what, and how to get the data back and keep it.
- Outputs Not EnergizingThe program says the output is on and the equipment is not running. Where the signal gets lost between the output image and the field device, and how to find the point in a few measurements.
- Inputs Not ReadingA switch closes in the field and the program never sees it. How the wetting voltage, the common, sinking and sourcing, and the wiring each lose the signal, and how to find which one did.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.