Can you explain how the variastor across the each DI, DO, AI, AO in MARK VIe system protects the system from voltage surges? How it works exactly?
MOV - Metal Oide Varistor.
An excellent discussion of MOVs (varistors) can be found here:
The main purpose is to prevent overvoltages from damaging other components in the circuit. MOVs are usually placed on I/O terminal boards across the input/output terminals to protect other, more sensitive cards--and I/O Packs--from damage.
The key thing to know about MOVs is that while they are like diodes in that they conduct above a certain threshold voltage, they conduct in both directions (positive and negative).
The other thing to know is that they break down over time, with no outwardly visible damage in many cases. What causes the break down? Constant and repeated surges in voltage. MOVs can only withstand so many surges and overvoltage conditions before they just "give up." And, this can occur with no visible signs of damage.
Whereas, in other conditions--severe and prolonged and extreme overvoltage conditions--MOVs can be burned and I've even seen them disappear from the terminal board (as in vapourized).
Under normal operating conditions, they don't do anything. There's just there for protection, and sometimes they are the sacrificial component.
Hope this helps!
Curious, though--why the question?
>The key thing to know about MOVs is that while they are like
>diodes in that they conduct above a certain threshold
>voltage, they conduct in both directions (positive and
MOVs are used to protect the Mark VIe printed circuit card traces and components against overvoltages. They are across the input terminals and when they conduct they are a short across the input terminals, protecting the traces and the circuitry on the printed circuits from damage.
MOVs are passive components--meaning that they aren't "controlled" by any other component or programming or circuit of the Mark VIe. When the voltage across the MOV exceeds the threshhold value of the MOV, it conducts--and since it's across the input terminals of the Mark VIe they are effectively a short across the input terminals. But, they don't conduct until their threshhold voltage is exceeded.
MOST control systems (of any type by any manufacturer) measure currents (0-1 mA; 4-20 mA; 0-20 mA; etc.) using a precision resistor across the input terminals (see the appropriate section of Mark VIe System Guide). For example, 4-20 mA through a 250 ohm precision resistor will develop 1-5 VDC. And, circuitry to measure voltage is much less expensive--and just as accurate-as circuitry to measure current. If the current in the circuit (passing through the "dropping" resistor (the precision resistor across the input terminals) greatly exceeds the design value, say it goes up to 1.0 A, then the voltage will increase to 250 VDC, which would probably cause the MOV to vapourize and disappear from the printed circuit card.
The MOV is an attempt at protecting the traces and circuitry on the Mark VIe printed circuit cards from damage due to excessive current, which would develop excessive voltage. They also protect the circuitry from the inadvertent application of high voltage to the circuit.