Interfacing Analogue and Digital
Analogue as Digital Inputs
With MODBUS now being a well established standard, most systems follow suit and relay most, if not all internal information using a 16-bit format. With MODBUS the ON state is when the value is equal to or above 32768 and OFF when 0 to 32767. If the analogue input on the device in question uses the 16-bit word to convey the analogue level such that the mid voltage/current range (e.g. 12 mA on a 4-20mA signal) equals 32768 or 8000H, then using the analogue as a digital input merely becomes a matter of ensuring there is sufficient current through the analogue input when ON is desired, and conversely, significantly low current to equal the OFF state.
Analogue as Digital Outputs
Analogue outputs are not that easy to interface to digital inputs but with a little trickery they can, in most cases, be successfully interconnected. As most digitals are relayed as 0000H for Off and FFFFH for On the analogue outputs will be driven to the limits of the range available on them.
Some outputs work from 0-20mA making interfacing extremely easy while others only operate from 4-20mA, the lowest current being more than most digital inputs require. A further problem is some outputs are 'alarmed' if the required current is not being drawn forcing them to either shut down or bombard the controlling system with a continuous warning.
The mechanics of interfacing analogues to digitals is discussed in the chapter "Building the System" under the heading "Wiring Analogues as Digitals".
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