### Lead/Standby The information below provides a FULL description of this microblock and all of its properties. What information you see and what you can do with it depends on your license and the application you are in, and in some cases, a controller's driver version.
Microblock family | Relay microblocks
Icon and symbol |
What it does | This microblock is used to control two devices, where one device is a standby (backup) to the other. It is commonly used to control critical devices, such as two pumps in parallel, because it will automatically turn on the standby device if the lead device fails. Swap outputs based on runtime On start-up, output o1 is the lead output. It turns on and off as the go input turns on and off. An internal runtime counter controls how long o1 remains as the lead output. When the counter expires, output o2 becomes the lead and turns on and off as the go input turns on and off. Output o1 then becomes the standby. This restarts the runtime counter. When the new runtime expires, o1 again becomes the lead and o2 becomes the standby. When go turns off, both outputs turn off without switching the lead and standby designation. Swap outputs when inputs swap The lead and standby outputs can also be switched by sending an on signal to the swap input. The outputs are not affected when swap turns off again. They remain switched until another event, such as an on signal to the swap input, causes them to switch again. You should only send a pulse signal to the swap input because leaving swap on prevents the microblock from responding to alarm inputs. Swap outputs based on alarm inputs These inputs signal a device failure and would switch operation to the standby device. For example, on a pump failure, the lead pump would turn off and the standby pump would turn on. The 2 alarm inputs, a1 and a2, correspond to the outputs o1 and o2. If o1 turns on but the device controlled by o1 fails to start, external control logic (not internal to microblock) should send an alarm signal to a1, causing the microblock to turn on o2 and turn off o1. Similarly, if o2 is on and a2 turns on, the microblock turns on o1 and turns off o2. Show lead status The output lstat indicates the status of the output currently designated as the lead. During normal operations, lstat remains on as long as the go input is on, even as the lead output switches between o1 and o2. If o1 is the lead and an alarm on a1 causes the output to switch to o2, lstat turns off because the lead output is no longer turned on.
Click for details below Properties Simulation
#### Properties TIPS Alt+click any value in the WebCTRL® or Field Assistant interface to view property details, including its editing privilege and expression (location path) for use on graphics. You can right-click some properties in the EIKON® Property Editor and select Make editable or Make read-only to determine that property's functionality in your system.
Reference Name RefName | Use the default reference name unless you want a more descriptive name for graphics or network links. Limitations: lower case only limited to 40 characters cannot begin with a number must be unique within a control program
Editing Privilege | Preset - Each microblock property has an appropriate privilege or role assigned to it. You can use Global Modify in the WebCTRL® interface to find out what the actual privilege is. CAUTION If you change the Editing Privilege from Preset, the privilege you select will be used for all properties of this microblock, which is not always desirable.
Swap based on runtime? Swap lead output after __ hours | If you want the standby output to become the lead based on runtime, check Swap based on runtime. In Swap lead output after __ hrs, set when the lead output's runtime expires. You can determine the starting runtime value using the Preset runtime value setting on the Properties page. For example, if the Preset runtime value is 100, and the Swap lead output after __ hrs setting is 150, the lead output becomes the standby output after 50 hours have passed (150 hours - 100 hours preset = 50 hours). Once the Preset runtime value is used, the Latch in preset value now property on the Properties page automatically changes to N. You must change this property back to Y to use the Preset runtime value again. NOTE If you manually swap using the input, the swap timer will be reset. TIP If you need to have at least one of the outputs on at all times, you may want to use Delay on Break microblocks on the outputs to account for the possibility of lag time when the outputs are swapped.
Property Page Text
Show Property Page Text | Check to show this microblock's value on the equipment's Properties page.
Property Page Text | You can edit the microblock description that appears on the Properties page. See "Editing Properties page text using special characters" and "Formatting a microblock property" in EIKON® Help.
#### Simulation Define the value(s) the microblock will use when you simulate the control program.