Test the panel link with the switch in RUN before you move it. Reads usually pass in RUN; writes can need TERM. If the switch stays in TERM, set bit 13 of V7633 so the CPU powers up in RUN, keep that register in the retentive range, and fit a memory card or battery for any outage past two weeks.
The panel manual tells you to put the PLC mode switch in TERM, and you do not want a machine that sits in STOP after a long power cut. Both halves of that problem have fixed answers. TERM is the switch position the serial documentation calls for when an operator panel has to write to the CPU; reads usually work with the switch in RUN, so test the link there first. If the write test fails and the CPU manual requires TERM, leave the switch in TERM and set bit 13 of V7633, the power-up in RUN bit on the DL05 and DL06. Then make sure that setting survives the longest outage your site actually takes. The steps below cover the test order, the bit and the retention hardware.
What the mode switch in TERM actually does
The switch position does two separate jobs, and mixing them is where the confusion starts.
Communications first. The serial protocol documentation for this CPU family names the TERM position for panel and programming traffic on the DL05, DL06, DL105 and DL205. In practice, reads from an operator panel usually succeed with the switch in RUN. The write is the operation that tends to demand TERM, which is why a panel manual can ask for a switch position you have never needed on read-only screens.
Power-up state second. With the switch in RUN, the CPU comes back in RUN after any power cut. With the switch in TERM, the CPU reads bit 13 of V7633 at power-up: the DL06 CPU specifications chapter documents it as power-up in RUN when the switch sits in TERM, and the DL05 carries the same register on this family. Bit set, the machine restarts running. Bit clear, a CPU left in TERM can sit in STOP after a long outage, which is exactly the failure you are worried about.
TERM does not stop a CPU that is already running. Once the scan is on, it stays on. The dropouts that do happen in service have a separate cause, covered at the end of this page.
What you need
- The CPU manual chapter on CPU specifications and operation, open to the power-up bit for your model.
- The programming software and its cable, so you can write one V-memory bit and read back the retentive ranges.
- A window with the machine shut down and guarded, so the write test cannot move anything.
- One spare, non-critical register for the panel to write.
- Backup hardware if the site can sit dark for weeks.
Set it up
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Leave the switch in RUN and read. With the machine shut down and guarded, connect the panel and display a value you have written into a spare register yourself. These CPUs talk serial; a DL05 offers two 6-pin RS-232 ports for programming and panel traffic, so the link is a serial cable. If the read fails, work the port before you touch the switch: protocol settings on both ends and, on a DL06 port 2 set from the ladder, the parameters in V7655 through V7657. The fix for no communications on one PLC model walks that check in order.
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Write once, on purpose. From the panel, write one value to that spare register. A working read tells you nothing about the write, and the panel earns permission to drive anything only after a write test passes. Follow the test writes before motion practice on any machine that is running.
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Stop here if both passed. Reads and writes in RUN mean the switch stays in RUN and the startup-state problem never arrives.
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Move to TERM on a printed requirement. If the write fails in RUN and the documentation for your CPU and protocol requires TERM, set the switch to TERM and leave it there as the permanent setting.
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Set bit 13 of V7633. From the programming software, turn on bit 13 of V7633 and leave every other bit in that register as you found it. The bit carries 8192 on its own, 2 to the 13th power, so OR it into the word rather than writing the word blind.
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Put V7633 inside the retentive ranges. A bit survives a power cut only inside the memory the CPU carries across the loss, so confirm the retentive ranges include V7633 while you are in the CPU memory configuration.
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Add hardware for long outages. If the site can sit dark past 14 days, fit a memory card on a DL05 or a D2-BAT battery on a DL06. The capacitor behind V-memory is a days-to-weeks buffer with no published hour figure, so the 14-day cutoff carries margin; the card or the battery holds V-memory for the full length of the shutdown.
How to check it worked
With the switch in TERM and bit 13 set, drop control power long enough for the CPU to reset fully, then re-energize. Run the test with the machine shut down and a recovery plan agreed, because the test is a real power cut. Three results to look for: the CPU comes back with the RUN indicator on while the switch sits in TERM, the panel read still works, and the panel write still works. If the panel comes back without its own screens or data, that is the panel's project storage, a separate fault with its own fix; a panel that loses its project after a power cycle is the article to follow for that.
Example: a DL06 left in TERM through a 24-day shutdown
Inputs. A packaging line runs a DL06 that writes setpoints from the panel, so commissioning left the switch in TERM after the write test failed in RUN. Bit 13 of V7633 is set, 8192 on its own, other bits untouched, and the retentive ranges include V7633. No card and no battery are fitted, so the capacitor behind V-memory is the only backup. The plant's shutdown history says the site can sit dark for 24 days. The planning cutoff from step 7 is 14 days.
Rule. V-memory keeps its values across a power cut only while its backup holds. Anything past the 14-day cutoff needs hardware backup: the memory card on a DL05, or the D2-BAT battery on a DL06, which holds V-memory as long as its charge lasts.
Arithmetic. 24 days ÷ 7 = 3.4 weeks. 3.4 weeks is 1.4 weeks past the 14-day cutoff (2.0 weeks), so capacitor-only retention is out for this shutdown. With the D2-BAT fitted, the backup covers all 3.4 weeks, so bit 13 is still set when power returns.
Result. Power comes back with the switch in TERM and bit 13 set, the exact combination the CPU chapter documents as power-up in RUN, and the line restarts on its own. Without the battery, the same 3.4-week outage sits past the cutoff, and the honest expectation is a CPU in STOP and a manual restart.
Decision. Dark longer than 14 days with the switch in TERM: fit the card or the battery and confirm bit 13 at the first restart. Dark 14 days or less: the capacitor with V7633 inside the retentive ranges carries it.
Why a CPU drops out of RUN in service
A CPU that leaves RUN while it is running points at one cause that shows up again and again: a large voltage spike, on the incoming power or on the communications wiring. The classic case is a CPU on an RS-485 network at a lightning-exposed site with grounding problems, where the same surges destroyed the RS-485 to RS-232 converters on the network. If a machine drops out of RUN in service, check surge protection and grounding on those two lines. The mode switch is not the cause, and moving it will not fix the dropout.
The decision
Recommendation: commission the panel link with the switch in RUN, test a read and a real write, and move the switch to TERM only when the write fails and the CPU documentation requires TERM. When TERM stays, set bit 13 of V7633, keep that register inside the retentive ranges, and fit a memory card on a DL05 or a D2-BAT battery on a DL06 for any shutdown that can run past two weeks.
Confirm the three settings together, switch position, startup bit and backup hardware, every time you commission or inherit a machine. A first-90-days PLC backup audit is the routine that catches the missing card and the tired battery before a long outage does. The screens mounted beside the line have their own needs; the shop-floor screens and andon boards section covers mounting, power and brightness for those.
Questions
Does the TERM requirement apply to the DL105 and DL205 too?
The serial protocol documentation names the TERM position for panel and programming traffic on the DL05, DL06, DL105 and DL205, so the switch requirement repeats across those four families. What does not carry over is the startup bit. Bit 13 of V7633 is the DL05 and DL06 address; the DL105 and DL205 document their own power-up bits in their CPU chapters. Find the printed bit for the CPU in front of you before you write anything into V-memory.
How long does the capacitor actually hold V-memory?
No published hour figure exists for these CPUs, and the working estimate is days to a few weeks, shrinking as the backup capacitor ages. You can measure your own unit: set bit 13 of V7633 on a spare CPU, remove power, and record the last day it still comes back in RUN with the switch in TERM. Cut that number in half and you have a cutoff with margin for the production machines.
What does the D2-BAT back up, and what does the memory card do?
A D2-BAT battery on a DL06 holds V-memory, including a retained V7633, for as long as its charge lasts, so date the battery when you fit it and check it on a schedule. A memory card on a DL05 stores V-memory without depending on a charge at all. Either one covers an outage far longer than the capacitor's days-to-weeks window.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.