Should I Purchase a 4208U or a 4208SN?

The 4208U is a universal 8-zone expander. Universal means that it is supported by panels that can use serial number zone programming and panels that support only dip switch mode. The 4208SN is also an 8-zone expander, but it is only supported on newer panels that support serial number mode.


The 4208U has been around for many years. Early versions of the Honeywell (Ademco) panels that supported V-Plex polling loop technology did so using devices whose zone number was determined by dip switch settings on the device. A dip switch setting of 12 meant that the device used zone number 12. With the 4208U, the device's dip switch setting determined which eight (8) zones the device would use.

If you notice in the chart below, dip switch six (6) is always set to OFF. This denotes that the device is being used in dip switch mode. "Loop Designations" is another way of saying "Zone Numbers". There are some disadvantages to this setup. A big one is that all eight (8) zones have to be programmed in numerical order. Also, since the VISTA panel hardwired inputs occupy zones 1 - 9, deciding which zone group to use could be tricky. An advantage of this setup is that it is very easy to program.



The early versions of the polling loop panels, such as the 4140XMP and VISTA-40 didn't support programming serial numbers for zones. These panels only supported the 4208U when it was set up using the chart shown above, and they couldn't support the 4208SN at all. As this lineup of panels progressed, they began to support both dip switch devices and serial number devices. These were panels such as the VISTA-50P, the VISTA-128B, as well as later polling loop panel versions. With the introduction of the Vista-128B, a feature called "Global Polling" came into use.

With Global Polling, the panel polls each polling loop zone individually for status, while simultaneously listening for any reports of a change of state from any of these devices. You can think of Global Polling as a teacher, taking roll in a classroom, albeit in a very fast manner. Quickly and efficiently, the panel takes roll, if a device needs to report a change of state, it does so in an orderly fashion, the change is processed, then the panel goes back to roll call.

Polling loop devices that use dip switch mode are not as well behaved as devices that use a serial number. Imagine a teacher attempting to perform roll call while some students, students whose names are not on the teacher's list, randomly stand up and shout out whatever thought is in their head at the moment. This is what it's like for a panel that is performing Global Polling while dip switch devices are in use.

Using dip switch devices, or devices such as the 4208U in their dip switch setting, will slow down and interfere with Global Polling. This can cause everything from slow response, to random trouble conditions. This is why if you use a 4208U on a system that supports the use of serial number polling loop devices, you MUST use it in serial number mode.

Now you're probably wondering, "What is this serial number mode of which you speak?" With the 4208U, if dip switch 6 is set to ON then the device will use serial number mode. What this means is, based on the setting of dip switches 2 - 5, when dip switch 6 is ON, the 4208U will use a particular set of eight (8) different serial numbers, one for each zone input on the device. You can see in the chart below what these serial numbers are.

Aside from the advantages of using serial numbers with Global Polling, setting up the 4208U this way also allows you to program any of the eight (8) zone inputs on any zone number you have available, with the exception of zones 1 - 9 (or 1 - 8 if you happen to be using a Fire/Burg panel). There are 16 different dip switch setting combinations, which means the 4208U can support up to 128 different serial numbers. If you happen to be using a VISTA-250BPT panel though, this may not be enough.


This is where the 4208SN comes to the rescue! With the introduction of the VISTA-250 panels, the 4208U was no longer able to provide enough serial numbers for the number of zones supported by the panel. The 4208SN was already around at this point, but was rarely used. Everyone was accustomed to the 4208U, and it allowed you to program the zones in dip switch mode, which was so much easier to program, even if it was wrong! Now, though, for very large installations that used more than 128 polling loop zones, the 4208SN was a must. The 4208SN supports 32 different dip switch setting variations, which means it will support up to 256 different serial numbers for each of the eight (8) zone inputs. It's able to do this because instead of using only dip switches 2 - 5 to determine which serial numbers will be used, it uses dip switches 2 - 7. You can see the dip switch settings and the corresponding serial numbers in the tables below.



So to recap, if you plan to be working with an older panel that only supports dip switch polling loop devices, you must use the 4208U, assuming you need an 8-zone expander. If you plan to work with any newer panel which will support serial number devices, you can use either the 4208U or the 4208SN, but if you choose to use the 4208U, you MUST use it in serial number mode with dip switch 6 set to ON.

If you plan to be working with a VISTA-250 panel and you will use more than 128 polling loop zones, you can use a combination of 4208U and 4208SN devices, as long as the 4208U is set up to use serial numbers for the zones. If you are buying all new equipment, though, we would recommend just using the 4208SN in this scenario, as this will cut down on the confusion of having different dip switch charts and serial number charts.

If you are replacing an older polling loop panel that could use only dip switch devices and it had a 4208U on it, and you plan to use the 4208U on a new panel that supports serial numbers, be sure to turn ON dip switch 6 on the 4208U, and program it to the new panel using serial numbers.

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