The Fault ouput is Open Collector, so provided there's not an issue with wires being connected wrongly, then you can connect it directly.
If noise is an issue, or the possibility of wires being connected wrongly, then having an opto would likely be beneficial.
The Fault output on the EM806 (i guess that's what you mean by the Alarm output) is a floating optoisolated transistor. You could do it two ways:
1. Flt+ to PIC Vcc, Flt- to the input pin and a 1k resistor from input to PIC ground. Input will go low on fault; or
2. Flt+ to PIC input, Flt- to PIC ground, 4k7 resistor from PIC Input to PIC VCC. Input goes high on fault.
Wouldn't using an internal pull up be just as easy?
Looking at the diagram in the manual again, I thought that if I use R = 4.7k, the PIC pin will see 5V when the FLT output is disabled, and 0V when the FLT output is enabled. With nothing passing through the FLT optotransistor in the EM806 ie. with no fault, the PIC pin should see 5VDC on the input, if the FLT optotransistor is enabled, it would pass current down to 0VDC, thus bringing the PIC pin down to 0VDC. Or am I missing something?
That'll work fine.
@m_c you could use internal weak pull_up but needs to be turned on in s/w and is only 10k so more susceptible to noise in this environment. Indeed on reflection I'd probably use a 1k external pull_up for just that reason.
Last edited by irving2008; 24-02-2014 at 05:35 AM.
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