When a newly added DL06 password makes an operator panel report PROTOCOL ERROR on every button press, the password format is the cause. Store it as a capital A followed by exactly 7 digits, such as A1234567: that invokes multi-level protection, which guards the ladder program while leaving the port open for the panel's writes. Removing the password restores the buttons but strips the protection.
Yes, this behavior is documented, and the fix is the format of the password. When button presses on an operator panel start reporting PROTOCOL ERROR right after you added a password to a DL06 PLC, the stored password has locked the communication port against the panel's ordinary writes. Store the password in the multi-level format instead: a capital A followed by exactly 7 digits, such as A1234567. That format invokes the multi-level password protection, which guards the ladder program while leaving the communication port open, so the panel's buttons work with protection still on. Removing the password also brings the buttons back, but it leaves the program open to anyone who connects. This page gives the symptom pattern, the checks that isolate the password as the cause, the exact format to store, and a worked example.
Why a password stops the panel's buttons
Chapter 3, CPU Specifications and Operation, of the DL06 User Manual describes 2 kinds of password protection. The standard kind is an 8-digit numeric password, and it locks the communication port. A button press on an operator panel is a plain runtime exchange: the panel writes a value and expects an answer, with no password step in between. Against a locked port that exchange fails, and the panel reports it as PROTOCOL ERROR on every press.
The second kind is the multi-level password protection. Chapter 3 states that it guards the ladder program while not locking the communication port, so runtime reads and writes keep flowing. The panel writes, the PLC accepts, and the program stays closed to any connection that has not entered the password. Same CPU, same cable, one character of difference in the stored value.
| Password stored | Protection type | Panel button writes | Ladder program |
|---|---|---|---|
| 8 digits, no letter, for example 48291307 | Standard | Fail | Protected |
| Capital A plus 7 digits, for example A1234567 | Multi-level | Work | Protected |
| All zeros, 00000000 | Removed | Work | Open |
Symptoms and what each points to
Match your symptom to one of these before you change anything. Each comes with the test that separates it from the others. The whole diagnosis takes about 10 minutes if the password is the cause.
- PROTOCOL ERROR on every press, starting right after a password was added or changed. The stored password has locked the port. Test: read the password back from the programming software and look at its first character. A digit first means the standard 8-digit format; a capital A first means multi-level.
- Displayed values froze at the same time the presses started failing. The whole port is blocked, which matches the standard password. Test: watch a value the PLC changes on its own, such as a cycle counter that ticks every 15 seconds of running time. If it stopped updating when the password went in, the connection is locked and the presses are only the most visible failure.
- One press fails, the rest work, and the values keep updating. That single write is the problem, most likely its target address or its value handling. Test: note exactly which press fails and what it writes, then check that button's target against the PLC addresses.
- The error predates any password, or survives removing it. The fault sits in the link, not in the credentials. Compare the panel's driver settings with the PLC port settings and check the cable. When the connection runs through a serial server, its setup joins the suspect list; the page on programming an HMI panel through an Ethernet serial server shows what that path adds. If the PLC link itself is in question, the article on an HMI programming link dead while the PLC link still works walks through separating the programming path from the runtime path.
Checks before you change anything
- Read the stored password back from the programming software instead of trusting anyone's memory. The first character decides which protection the CPU applies.
- Count the characters against the table above: 8 characters in total. A digit first locks the port; a capital A first leaves it open.
- If the line is down and you are authorized, remove the password temporarily with 8 zeros, 00000000, and press a panel button. If it works with the password gone, the password was the block. Removing the protection leaves the ladder program open to anyone who connects, so keep this window under 15 minutes and close it in the same visit.
- If the error persists with no password stored at all, stop editing credentials and work through the link checks instead. Repeated password changes only muddy the record.
The fix: store a capital A followed by exactly 7 digits
- Connect from the programming software with the current password, under your site's authorization. If the password is lost, use the recovery procedure the DL06 User Manual documents; do not guess entries.
- Change the stored password to a capital A followed by exactly 7 digits, no space between them: A1234567 is the shape. The whole password is 8 characters, one letter and 7 digits.
- Write the password to the PLC and check that the programming software still demands it before the ladder program opens. That check takes under 1 minute and shows the protection survived the change.
- Press each panel button that failed, at least 3 test presses, and watch the value each one writes in the PLC. A moving value shows the write path came through the change too.
- Hand the password to authorized staff through your site's credential process. Keep it off the screen, off labels and out of the panel project.
Worked example: a filling line writing again by the next shift
A March audit leaves a filling line's DL06 protected with the password 48291307. Read its 8 digits: 4, 8, 2, 9, 1, 3, 0, 7. Eight digits, no letter, first character 4, so the CPU stores the standard protection and locks the port.
Next shift, 8 hours later, the operator presses CYCLE START and the panel answers PROTOCOL ERROR. The batch counter shown on the panel has not moved since the audit either, so the whole port is blocked. That matches the standard password.
An authorized technician stores 00000000 and presses CYCLE START within 5 minutes of the report. The line starts, so the password was the block. Protection is now off, and the technician moves straight to the permanent change in the same visit, keeping the unprotected window under 15 minutes.
The technician stores A482913. Read its 8 characters: capital A, then the 7 digits 4, 8, 2, 9, 1, 3, 0. One letter plus 7 digits, 8 characters in total, which is the multi-level format. CYCLE START writes from the panel, the batch counter updates again on the next count, and the programming software still asks for the password before the ladder program opens. Total downtime for the credential part: about 20 minutes. Protection on, buttons working, nothing else on the line touched.
Verify the writes, not just the screen
A panel can paint a healthy screen while a button exchange still fails, so test the presses that failed under a condition where a stray write cannot damage product. Watch the PLC side of each press: a momentary press holds its bit for only a few PLC scans, on the order of 2 to 3 scans, so watch the value live in the software or log it. The page on how many PLC scans an HMI momentary button bit stays on shows what a short press looks like from the PLC side. Repeat for every press that failed, then record the tested presses, the date and the password format in the maintenance log. Record the format, never the password itself.
Recommendation: On any DL06 an operator panel writes to, store the password as a capital A followed by exactly 7 digits, such as A1234567. Reserve the plain 8-digit numeric password for PLCs that no panel connects to, and use the all-zeros removal only to verify the diagnosis or hold production, closed out in the same visit.
Questions
Does the multi-level password work with the LCD display panel on a DL06?
No. The DL06 User Manual states that the LCD display panel does not support the multi-level password. That option module keeps its own LCD password, and only menu 5 on the LCD display itself changes it. If your machine carries one of these modules, treat the PLC password and the LCD password as two separate settings, and test the panel writes after you change either one.
Where is the password format documented?
In the DL06 User Manual (D0-06USER-M), Chapter 3, CPU Specifications and Operation. The chapter states that the CPUs support multi-level password protection of the ladder program and that this protection leaves the communication port open. It also covers the 8-digit numeric password and the all-zeros removal value. The multi-level format note sits in the password protection part of the chapter.
Does changing the password format touch the ladder program or the panel project?
No. The change is one stored value in the CPU. The ladder program is not edited, the panel project is not edited, and no address or driver setting moves. The follow-up work is handing the new password to authorized staff through your site's credential process and updating any runbook that still prints the old value.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.