Historically Host Applications have polled HART field devices when digital integration is supported. Even though most I/O now has one modem per channel, communications bottlenecks occur in the I/O. One challenge for I/O is determining how often to send Command 48 once the More Status Available bit is set. Worse case every other command is Command 48 when the More Status Available bit is set. This dramatically reduces the bandwidth available for runtime process data and instrument management communication.
To address this substantial communication bottleneck, the ability for a host to reset the More Status Available bit was added in 2007 with HART 7. Support for resetting the More Status Available enables masters to poll Command 48 by exception, rather then continuously. To enable this the field device must SET the More Status Available bit when any status changes AFTER the More Status Available bit is RESET (i.e., for both on 1->0 and 0->1 changes in status). Only doing this allows the objective behind allowing the More Status Available bit to be reset be achieved. (Note: Only changes in status that affect More Status Available will change its value. Not all status contained in Command 48 must affect More Status Available.)
More specifically, the More Status Available bit shall be SET until either:
(a) The master resets the bit (by writing the correct pattern); or
(b) The reason for setting the bit is not present anymore.
In the first case (a), the master reads Command 48 to update its cache. Then the Command 48 data is written back to the Field Device to confirm the cached values match those in the Field Device. It they do then the Field Device resets More Status Available.
In (b), when the status triggering More Status Available is cleared More Status Available is RESET. This is how More Status Available has historically behaved. In other words, a HART 5/6 master will see no change in device behavior.
Detailed behavior of Command 48 status and More Status Available is summarized in Table 4. The key to setting/resetting More Status Available is detecting a difference between the current value of Command 48 data and the value when the master last RESET More Status Available. Typically, this requires the Field Device to maintain a copy of the Command 48 data for each master.
For the bits that affect More Status Available, a difference between the copy and current Command 48 data means More Status Available must be SET. Resetting More Status Available results in the current Command 48 data being written to the copy.
Table 1. More Status Available State Transition Matrix
| Initial Values |
Event |
Final Values |
||
| MSA |
DSS |
MSA |
DSS |
|
| 0 |
0 |
DSS is set (Something happened in the device) |
1 |
1 |
| 1 |
1 |
DSS is reset (DSS value is same as last time MSA==0. The DSS cleared before the master confirmed its reception of Command 48 response.) |
0 |
0 |
| 1 |
1 |
MSA is reset (Device received a Command 48 request that contains an exact match for the status data in the Field Device. This means master has acknowledged reception of current Command 48 values.) |
0 |
1 |
| 0 |
1 |
DSS is reset (Device issue cleared but Command 48 response no longer matches what was last acknowledged by the master.) |
1 |
0 |
| 1 |
0 |
DSS is set (DSS value is same as last time MSA==0. DSS returns to state last acknowledged by the master.) |
0 |
1 |
| 1 |
0 |
MSA is reset. Device received a Command 48 request that contains an exact match for the current status data |
0 |
0 |
MSA = More Status Available
DSS = Device-Specific Status. Device Specific Status is any status bit in Command 48 that asserts More Status Available.