Douglas Krantz - Technical Writer - Describing How It Works
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What is Capacitive Lag?

Capacitive lag is the lag in time needed to charge the capacitance between the wires. Capacitive lag isn't a time delay of the undistorted signal; it's a distortion of the data signal.

A clean data signal will not show any capacitive lage. In real life, there's always at least a small amount of capacitive lag.
If there weren't any wire between the digital transmitter and the digital receiver, the signal would look like this. In order to read the transmitted signal properly, the receiver would not have a need for a threshold: a "1" would be properly received, and a "0" would be properly received.


By Douglas Krantz Douglas Krantz

One of the biggest issues that limit the length of wire used in a Signaling Line Circuit (SLC) is the amount of Capacitive Lag. If the wire length is too long, the capacitance of the wire will corrupt the data signals, and prevent proper communications between the panel and the devices.

At first glance, one would think that it's a distance-between-the-panel-and-the-module, but it's not. It's an issue of the total capacitance of the wire. T-tapped 10 times, or a single straight segment, it's the amount of wire, not the distance that matters.

Wire Capacitance and Signal Generator Resistance

A capacitor is defined as "Two Conductors Separated by an Insulator". Wire is a capacitor, wire also has resistance. And sending a signal through the wire, the power supply (digital transmitter or signal generator) has resistance.

All wire has some capacitance, and there's always at least a little resistance to the current flowing into or out of a capacitor



This means that the almost instantaneous voltage change that is sent by a digital transmitter takes time to charge or discharge the wires carrying a signal.

Threshold

Whether it's wire or an electronic component, a capacitor takes time to charge or discharge

A capacitor takes time to change voltage because it takes time to put electricity into the capacitor, or time to take electricity out of the capacitor.

The time it takes for the voltage in the capacitor to cross a threshold, either charging or discharging, is the capacitive lag. Remember, the wires are a capacitor.

Why is there a threshold?

The threshold is less than the absolute "1" and greater than the absolute "0" voltage levels for three major reasons:
Most of these problems can be least mitigated by the data receiver. The data receiver stays at "0" until the voltage crosses the "1" threshold, and then the data receiver stays a "1" until the voltage crosses the "0" threshold.

Capacitive Lag Time Compared to Digital Bit Time

How much the data is affected by capacitive lag is relative. The question is how long the capacitive lage is compared to the bit-time length.

Compared to the digital signal being transmitted:
Once the lag (as measured by the time it takes for the voltage to cross the threshold) exceeds a bit in time length, that's it. There is no more data. The data is now corrupted and may as well as not have been sent.

Capacitive Lag

Partially because of capacitive lag, the receiver accepts some imperfection in the data signal.

This lag in time, between when the data signal changes from a "1" to a "0", or a "0" to a "1", is always on the wires between the transmitter and receiver.

Compared to the data-bit length, though, if the lag is short enough, the data will be received correctly. If the lag is too long, some or all of the data-bits won't be received, and the data becomes corrupted.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer
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facpdoug@douglaskrantz.com
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