How to Activate Output with TOF Timeout and DN Bit Issues

Years ago, in RSLogix, a warning was issued regarding the use of TOF timers. It is crucial to anticipate all potential scenarios, including operator error, and ensure that the TOF timer is properly reset after an emergency stop. Failure to do so could result in the immediate activation of various outputs such as pumps, conveyors, mixers, or valves, potentially jeopardizing the safety of individuals in the vicinity.

• 08-07-2024
• I_Automation

In the world of automation, there have been warnings in the past regarding the use of TOF timers in RSLogix. It is crucial to consider all potential scenarios and reset the TOF timer to avoid any dangerous situations after an emergency stop. Failure to do so could result in the immediate activation of equipment such as pumps, conveyors, mixers, or valves, posing a risk to individuals in the vicinity. One important factor to take into account is the PLC transitioning into RUN mode. If the value of Input Local:2.I.Data.2 is not 1 during the first scan (S:FS value is 1), the PLC may not accurately track the duration of a 0 signal for 2 seconds. This could lead to an undesired output being turned on erroneously, potentially causing harm. By using the S:FS First Scan bit's value, this issue can be mitigated by delaying the activation of the output for 2 seconds after the PLC enters RUN mode, even if Input 2 is initially off. Alternatively, incorporating a TON driven by XIO Local:2.Data.2 in the logic can provide a cleaner solution to handle this particular scenario. The choice between using a TOF or a TON should be based on what makes more sense for the individual who will be troubleshooting any process malfunctions. The TOF logic signifies a delay in turning off, while the TON logic signifies a delay in turning on, catering to different operational needs.

• 08-07-2024
• drbitboy

In response to MrRobotguy's comment expressing gratitude for the helpful information, it is important to note that a Time-Off Delay (TOF) is necessary due to consistently high input levels in the application. If the input remains broken for 2000 milliseconds, the DN bit is triggered. While most TOF applications can be represented by Time-On Delay (TON) functions, it may not always be required, as exemplified by Ken Roach.

• 08-07-2024
• durallymax

In a discussion by MrRobotguy, it was highlighted that the selection between TON and TOF timers should be based on the output state rather than the input signal. A TON timer is ideal for delaying the act of turning something on, while a TOF timer is best for delaying the act of turning something off. In this scenario, where the output needs to be activated after a 2-second delay, opting for a TON timer is the most suitable decision. The fact that the input signal is consistently high indicates that an XIO instruction should be used instead of an XIC. While interchangeable, utilizing one instruction over the other may lead to more complex logic or create unique situations like the one highlighted by drbitboy in the previous discussion.

• 09-07-2024
• plvlce

While plvlce may have beaten me to the punch, I still want to share some valuable information on Rockwell timers. To simplify, remember this rule: use the TON timer to delay turning something on, and use the TOF timer to delay turning something off. The key is to focus on the action you want to take, rather than whether the input triggering it needs to open or close. Additionally, utilize the DN bit if you want a certain action to occur after a specified amount of time, and the TT bit if you want it to occur for a specific duration. For example, if you need to turn on an output two seconds after an input opens, use the TON timer to make something turn on after two seconds (DN). This will ensure efficient control over your operations.

• 10-07-2024
• Operaghost