DIGITAL INPUTS (TTL LOGIC INPUT TO THE SOCKET):
38 Din 1 Mains On (single pulse)
39 Din 2 Mains Off (single pulse)
40 Din 3 Feed Pump On (level)
41 Din 4 Hot Water On (level)
43 Din 5 SSR Remote
44 Din 6 Spare
45 Din 7 Remote Operation Request
46 Din 8 Watchdog Pulse (for remote operation)
26,27,32,37,42,47,48 Digital Ground
Remote Control Operation
The FT74 may be controlled remotely from a computer or a PLC. When in remote
operation, the control panel functions are disabled (except the emergency stop). To
enter remote control mode, the Remote Operation Request bit must be set high, and a
‘Watchdog’ pulse waveform applied to bit 8.
The use of a watchdog circuit is in line with industrial practices, for safe control of
processes by computers and other programmable devices. A monitoring circuit is
fitted in the electrical tray, which monitors a dedicated digital line from the
controlling device for a regular programmed pulse (watchdog pulse). Should the
program stop running or the controlling device become disconnected, the watchdog
circuit will turn off remote control facilities and revert to manual control.
The watchdog pulse must be a logic signal of TTL compatible level and of repetition
rate less than 8 seconds and mark or space width greater than 20 milliseconds. A
typical watchdog pulse has a 1 second mark and a 1 second space.
56
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