Douglas Krantz - Technical Writer - Describing How It Works
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What is the Typical Clock Rate of an SLC?

The Baud Rate for an SLC can be anywhere from 300 Baud (Bits Per Second) to 9600 Baud. There is no typical or standard baud rate, clock rate, voltage, or protocol for Signaling Line Circuits (SLCs).

What is the Typical Clock Rate of an SLC?


Greetings Douglas,

I've had a really tough time trying to get an answer to this question - what is the typical clock rate of a Class B SLC Data Bus?

Based off my observation of the poll rate of devices on the SLC, I have to imagine it's fairly slow, possibly in the kHZ.

I've tried to find a standard or manual with this info but so far, no luck. Unfortunately, I no longer have access to an oscilloscope!

Thank you, EG

The short answer to your question is that for fire alarm system Signaling Line Circuit (SLC) data buses, there is no such thing as a typical clock rate. Each manufacturer uses a different baud rate, voltage, and data-protocol on their SLC circuits. To make it even more complicated, many manufacturers use several different baud rates, voltages, and data-protocols.

The baud rates range anywhere from 300 baud to 9600 baud. A few SLC circuits even have more than one data baud rate.

The SLC is a Data and Power Carrying Circuit

The SLC isn't just a data-only carrying circuit like an RS485; the SLC also carries power to operate the addressable portions of all of the devices attached to the SLC - like input modules, control relay output modules, and supervised output modules. This power is also used for detectors that are included with some input modules.

With industrial process controls, on the data-bus (commonly RS485) there is a standard baud rate, and a standard protocol for the data going from the control panel to the devices. Fire alarm systems are different, they are ALL proprietary. Fear of litigation is the reason; no manufacturer wants to be held accountable for another manufacturer's failed equipment.

A Class A SLC circuit and a Class B SLC circuit are exactly the same, except that a Class A SLC circuit has both the beginning and the end inside the control panel, and a Class B SLC circuit has the beginning of the circuit inside the control panel and can have multiple endings in multiple parts of the building.

Capacitive-Lag Limitations

Checking with manufacturers, they will tell you the limits available on the total amount of wire for each SLC circuit. The limits are because of capacitive-lag: the lag in time due to the RC (Resistive/Capacitive) charge and discharge rate of the wire.

Because a capacitor is two conductors separated by an insulator, just the wire itself is a capacitor. The capacitive value of the wire-capacitor is determined by the wire diameter (which affects the plate area of the wire-capacitor), both the thickness and the dialectic constant of the insulation, and the total added up length of the cable. If there's a shield, the shield will also affect the capacitance.

The power of the data signal does not "travel" along the cable of the SLC like one would expect with a transmission line like an RS485 circuit: there is no terminating resistor. Instead, the capacitive lag determines the fastest baud rate. If the total wire, including T-taps, is too great, the capacitive lag of the wire will corrupt the data signal.

Power Supply Limitations

The power supply for the SLC is extremely weak, it has a very high resistance. This weakness in the power supply is for the purpose of data communications with the devices in the field. This high resistance, though, means that the RC (Resistive/Capacitive) charge and discharge of the wire corrupts the data if the baud rate is very high.

This explains why the observed baud rate is in the KHz range, or slower.


I'd like to point you at a good source of information, but because every manufacturer is proprietary, there's no really good source anywhere.

Douglas Krantz

facpdoug@douglaskrantz.com
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