Discrete Monitor (DM) and Discrete Control (DC) Points
Explanation of Discrete Monitor (DM) and Discrete Control (DC) Points
These points are always used in a pair. They control devices that would otherwise be controlled by an ordinary DO but are considered critical enough to warrant a DM/DC combination.
The DC point does the actual controlling (opening and closing of the hardware contact) and the DM point provides positive feedback from an external discrete device (for example, an air flow switch). The DM point is typically wired to a proof-of-flow switch that transitions when the controlled device is started or stopped.
It may also be used to monitor an auxiliary contact on a motor starter if a proof-of-flow switch is not installed. However, used in this way, the only information you are really receiving is that the contacts have closed or opened, but not whether the device is actually running.
Adding a resident DM point to a controller requires 30 bytes of memory.
Adding a resident DC point to a controller requires 37 bytes of memory.
When controlling DC/DM points using CU/PCU resident DDC, calculated points, or automatic temperature control (ATC), the Resident I/O Points editor entries of Scan Interval, Time to State, and Alarm Delay are very important.
Use the guidelines below to ensure that an alarm is correctly generated any time the DM point’s actual state conflicts with its respective DC point’s Expected State:
See Link Below
Regardless of which program is used, the time to state and alarm delay entries for the points should always be set large enough that the point being controlled is allowed ample time to change state (start, stop, etc.) before the point is declared to be in alarm. The DM point’s alarm delay value should always be set greater than the DC point’s time to state value.
In DCU/PCU resident DDC, the scan interval of the module should be set greater than the time to state and scan interval entries of the DC point being controlled.
When using a calculated point extension to drive a DC/DM pair, the DC point’s time to state should be less than the scan interval of the point.
When using ATC to control a DC/DM pair, it is important to remember that the point will always be issued a command by the program at the rollover of each minute. If the space temperature exceeds the cooling setpoint plus 1⁄2 the differential at 35 seconds past the minute, the DC point will be controlled ON in 25 seconds, again 60 seconds later, again 60 seconds later, and so on. When using ATC, it is recommended that the scan interval, alarm delay, and time to state entries all be set less than 60 seconds, allowing this alarm checking to be completed prior to the rollover of the next minute, when ATC issues its next command.
For further detail please use following link: DC-DM Operation.doc