Measurement Computing CB-7000 Utilities - Manual (Page 24)

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3 Dual Watchdog
3.1 Operating Principles
All 7000 modules are equipped with a hardware module watchdog and software host watchdog.
The 7000 series are designed for industrial applications. Therefore, they can work in a harsh
environment. However, there are often high-voltage transients in such environments. The
modules may go down if noise levels are too high. The built-in hardware module watchdog can
reset the module if it is down for too large signal (refer to Figure 3-1). Sometimes even the host-
PC may be down for hardware or software reasons. The software host watchdog will monitor
the status of host-PC. If the host-PC is down, a ll the outputs of 7000 modules will go to their
predefined safe states. (Refer to Figures 3-2 to 3-3.)
If the RS-485 network is open, host commands cannot be sent to remote modules. This can be
dangerous in some applications. To prevent a serious problem, the 7000 output modules will
force their outputs to pre-defined safe states if the host watchdog is active. This dual watchdog
feature increases the reliability of system as follows:
Dual watchdog = module watchdog + host watchdog
Host watchdog
: (software)
If the host is down, all module outputs go to a safe state (Safe value). See Figures 3-2 and 3-3
Module watchdog
: (hardware)
If the module is down, it resets the module; the output goes to a pre-determined Power-On value.
(See Figures 3-4 and 3-5.
3.2 Host Watchdog
The host can go down for the following reasons:
Noise signals are too high. This causes host hardware to make errors.
Software problem. This makes the host go to the dead-lock state.
Hardware problem; host hardware is damaged.
The RS485 network is open and can’t send out commands to modules.
The software host watchdog monitors the host computer. If the host computer is down, the
output of the 7000 modules will automatically go to their safe states to avoid possible
process damage.
When the Host PC periodically sends command “~**”, it signals that the Host PC and wires are
OK (Figure 3-3).