When the computer is turned on, all devices connected to an Amplicon PC272E are turned on. This can be dangerous.
Why does this problem occur?
When the Amplicon PC272E cards power up, they reset their 82C55 controller chips. When the 82C55 chip resets, it places all its lines into input (high-impedance) mode. When a line is set to input mode, a set of pull-up resistors on the PC272E card bring the voltage high, and if an output device is connected to this line, it will be turned on. These resistors are not configurable (unless you feel like resoldering the board), and the net result is that the outputs stay on until the software reconfigures the lines for output. Sources: Peter Adams (Amplicon Technical Support, tel. 01273–601331) and the Appendices for the 71055 and 8255 controller chips on the Amplicon CD-ROM.
These are not valid solutions:
| • | Swapping all outputs from the "normally open" to the "normally closed" relay pins. In this situation, short power cuts (which reset the computer) are safe, but now long power cuts (which may leave certain computer motherboards switched off) are dangerous: the box power supply comes on but the computer stays off. |
What are the possible solutions?
| • | Use different digital I/O cards. [Impractical for us.] |
| • | Rewire the existing cards. [Difficult.] |
| • | Use a motherboard that guarantees never to power on unsupervised. [Very hard to guarantee.] |
| • | Plug the computer and/or DC power supply into a trip switch, so a mains power cut cuts power to everything until reset by a human. [Possible with an "industrial self-testing plug", £48 each from RS, but not the best solution.] |
| • | Wire the DC power supply for the critical devices through a fail-safe mechanism. Probably the best is to wire it through two relays such that one relay must be ON and the other must be OFF in order for power to reach the devices. This is extremely unlikely to happen by chance (all the problems described so far switch all the relays on or off together). The server software provides support for assigning some relay outputs to this function. The only criterion that must be fulfilled is that the relays can take the current: the relays on the EX213 board can switch AC to 60VA and DC to 28W (i.e. 1A at 28V). [This is a very good solution, and can be combined with an UPS.] |
Further to this: the relays on the EX213 board will take 8A, but the tracks on the circuit board will probably handle only 1A (Russell English, Amplicon tech. support, pers. comm. 1 Dec 2000). This is enough for individual devices, but not for the entire power supply for a system. The TTL outputs on the EX233 distribution board will provide about 2.5 mW; the high-voltage (24V) logic outputs on the EX213 board will provide 100 mW; the TTL-capable outputs on the EX213 board will probably provide 100 mW too (though nobody seems absolutely convinced of this). As a typical 10A relay takes about 250 mW to drive its coil (so approx. 10 mA at 24 V; coil resistance approx. 2.4 kW), we opted for a high-power relay that is itself switched by a relay on the EX213 board.
| • | Install an UPS for the computer. This eradicates the problem and gives data protection. If you're willing to spend money on a very powerful UPS, you could also guarantee DC power to the operant chambers, so your tasks would not even be interrupted by a power cut. [This is a good solution and can be combined with the fail-safe relay device.] |
See also