Say you have a 16/35 PowerLab and you want to use all sixteen channels. You want to measure Bio Potentials on 10 of them, and use a multi-purpose amplifier such as the Bridge Amp on the remaining six. The following image shows the I2C (9 pin) connection and highlights which Front-End inputs are used. The BNC connections from the Front-Ends to the PowerLab inputs have been omitted.
You’ve got two Octal Bio Amps, and one Octal Bridge Amps. That’s a total of 24 potential channels, so a possible combination that totals to 16 is 10 Bio Amp channels plus 6 Bridge Amp channels. Unfortunately, this setup will not produce the desired result. To understand why this is, we need to introduce the concept of slots.
Each PowerLab has a fixed number of I2C slots (see list below). When a Front-End is connected, it takes up slots. It’s most convenient to think of a Front-End using up slots as it connects to the PowerLab. The number of slots a Front-End uses up is independent of the number of inputs that are actually used. So, even if using only two input channels of an Octal Bio Amp, it still uses up eight slots. This is the source of the misconception, and unfortunately inherent to the I2C daisy-chaining!
Let’s examine the example of 10 Bio plus 6 Bridge channels that I said wouldn’t work.
All 16 channel PowerLabs have 16 slots to allocate. Now, let’s follow the daisy chain step by step: The first 8 Bio channels are straight forward: 8 inputs take up 8 slots. The second Bio Amp only uses 2 inputs, but it also takes up 8 slots. So, there are no slots left over for the Octal Bridge Amp, even though there are still 6 PowerLab inputs unused!
Because all of the Front-Ends in this example take up eight slots, changing the order of the Front-Ends doesn’t help. Fortunately, in most cases ...
The order matters!
If the PowerLab has nearly run out of slots, it will allocate as many as it can and then stop. Back in the reality of your lab, this means that if you have a many-channel Front-End that you don’t want to use all inputs of, you’re best off placing it last in the daisy-chain. This is because surplus slots requests are ignored by the I2C protocol.
In the next image a Quad Bridge Amp and two Octal Bio Amps are connected to get 12 Bio Amp channels plus 4 Bridge Amps.
How many slots does my Front-End use up? Each type of PowerLab has a set number of external slots.
The number of slots a Front-End requests are usually the total number of channels it has, but not always. All single channel Front-Ends use one slot. Some devices that connect with I2C don’t use up any slots, because they only draw power.
Multiple PowerLab solution
If your experiments are likely to use many Front-End amplifiers, it is worth contacting your ADInstruments Representative and considering the option of using multiple PowerLabs (with LabChart for Windows 7.2 or later). In my example above, with two Octal Bio Amps and one Octal Bridge Amp, an extra 8/35 PowerLab makes the desired combination (10 Bio plus 6 Bridge Amp channels) possible – along with many more! Using LabChart for Windows 7.2 and up, up to 32 channels can be sampled into the same document. You can re-arrange the PowerLab inputs to appear in any channel order you require by assigning them in the Devices And Channels dialog.
The 8/35 and 16/35 have sixteen slots each. Note that any Front-End unit can only be connected to one PowerLab. Its channels cannot be shared across multiple PowerLabs.