RSLogix 500: Moving Timer ACC into Another Timer PRE
RSLogix 500 timer programming: capture a running timer's ACC and transfer it to another timer's PRE for reversible door simulation with safety beam.
Bidirectional motion emulation in RSLogix 500 often needs a timer that runs for the time already spent traveling in the opposite direction. A garage-door trainer is the classic case: if the photo-eye trips after three seconds of down travel, the reverse up-travel must run for three seconds, not the hard-coded six-second preset. That requirement cannot be met by editing the source timer’s accumulator. The live .ACC word is overwritten by the timer instruction every scan while the rung is true. The durable pattern is to capture .ACC into an integer on a one-shot edge, then write that integer into the destination timer’s .PRE before enabling reverse travel.
Read .ACC, stage through N7, write .PRE—never treat the accumulator as a lasting setpoint.
Timer file architecture
Each SLC timer occupies three words: status bits (EN/TT/DN), .PRE, and .ACC. Timebases are 0.01 s, 0.1 s, or 1.0 s. On a 0.01 s base, a preset of 600 equals six seconds. Publication 1747-6.15 documents the instruction set behavior in detail.
| Element | Role | Writable setpoint? |
|---|---|---|
| T4:x.PRE | Target count | Yes — correct destination |
| T4:x.ACC | Elapsed count | No lasting effect while EN true |
| T4:x/TT | Timing qualifier | Use to gate capture |
Why MOV into .ACC fails
While EN is true, the TON/TOF/RTO instruction updates .ACC every scan. A user MOV into .ACC is overwritten on the next (or even same) scan. Reloading .ACC also does not redefine the comparison target; the instruction compares .ACC against .PRE. Writing a new .PRE and restarting from .ACC = 0 is the controlled way to change duration.
Capture with ONS, transfer with MOV
Rung capture: XIC Door_Going_Down XIC Down_Timer/TT ONS Capture_Bit MOV T4:2.ACC N7:10 Rung load preset: XIC Capture_Bit MOV N7:10 T4:3.PRE Rung latch: XIC Capture_Bit OTL Preset_Loaded Rung default (gated): XIC Door_Idle XIO Preset_Loaded MOV 600 T4:3.PRE Rung clear: XIC Up_Timer/DN OTU Preset_Loaded
RSLogix 500 will not accept a MOV that uses the same timer element as both source and destination. Always stage through an integer file word such as N7:10. The ONS storage bit keeps the capture on the leading edge of the beam so a held input does not rewrite the preset every scan.
State machine around the transfer
| State | Entry condition | Action |
|---|---|---|
| IDLE | Power-up / both done | Clear one-shots and direction bits |
| GOING_UP | UP edge | Default up preset if not loaded |
| GOING_DOWN | DOWN edge | Default down preset |
| REVERSING | Beam edge while down | Capture ACC → up PRE, enable up |
Studio 5000 migration notes
Logix timers use the same .PRE / .ACC semantics with tag-based addresses. Capture into a DINT, write the reverse timer’s .PRE, and retain one-shot edge detection with ONS or equivalent. Knowledgebase A-43402 discusses moving accumulated values into other tags on ControlLogix; the element roles match the SLC pattern.
ACC capture and PRE write edge checks
Trip the beam at a known elapsed time and confirm the reverse preset equals the captured ACC within one timebase tick. Confirm default presets only load when Preset_Loaded is false. Common mistakes include writing .ACC, omitting the integer stage, capturing without /TT qualification, and allowing an initialization rung to overwrite .PRE after capture. Keep timer documentation consistent with other PLC and PAC standards when migrating the trainer logic to Logix.
About the Author
Mark Townsend | Senior Automation Engineer – Allen-Bradley Systems
Mark Townsend is a senior automation engineer with more than 18 years on Allen-Bradley platforms spanning ControlLogix, CompactLogix, and legacy SLC-500. His day-to-day work is RSLogix / Studio 5000 logic and FactoryTalk View HMI bring-up on aging and mixed fleets.
