a BIST mechanism is provided in advanced fieldbus systems to verify functionality. At a high level this can be viewed similar to the PC BIOS's power-on self-test (POST) that performs a self-test of the RAM and buses on power-up. Computers The typical personal computer tests itself at start-up (called POST) because it's a very complex piece of machinery. Since it includes a computer, a computerized self-test was an obvious, inexpensive feature. Most modern computers, including embedded systems, have self-tests of their computer, memory and software. Unattended machinery Unattended machinery performs self-tests to discover whether it needs maintenance or repair. Typical tests are for temperature, humidity, bad communications, burglars, or a bad power supply. For example, power systems or batteries are often under stress, and can easily overheat or fail. So, they are often tested. Often the communication test is a critical item in a remote system. One of the most common unattended system is the telephone concentrator box. This contains complex electronics to accumulate telephone lines or data and route it to a central switch. Telephone concentrators test for communications continuously, by verifying the presence of periodic data patterns called frames (See SONET). Frames repeat about 8,000 times per second. Remote systems often have tests to loop-back the communications locally, to test transmitter and receiver, and remotely, to test the communication link without using the computer or software at the remote unit. Where electronic loop-backs are absent, the software usually provides the facility. For example, IP defines a local address which is a software loopback (IP-Address 127.0.0.1, usually locally mapped to name "localhost"). Many remote systems have automatic reset features to restart their remote computers. These can be triggered by lack of communications, improper software operation or other critical events. Satellites have automatic reset, and add automatic restart systems for power and attitude control, as well. Medicine Medical devices test themselves to assure their continued safety. Normally there are two tests. A power-on self-test (POST) will perform a comprehensive test. Then, a periodic test will assure that the device has not become unsafe since the power-on self test. Safety-critical devices normally define a "safety interval", a period of time too short for injury to occur. The self test of the most critical functions normally is completed at least once per safety interval. The periodic test is normally a subset of the power-on self test. Military One of the first computer-controlled