The MUX board v2024 uses the same technology than the MUX board v2023, i.e. mechanical multiplexing. However it introduces
a new level of flexibility by addressing multiple roles (A, B, M, N), which allows to build OhmPi system with multiple of 8 electrodes.
The multiplexing of the channels is a mechanical multiplexing based on OMRON's manufacturing relays (G5LE-1-VD 12 VDC). Each relay is combined with
Users can physically configure the MUX board to address 2 roles (A, B or M, N) or 4 roles (A, B, M, N). With only 32 relays,
a ZVN4206A power MOFSET. The raspberry has only 30 GPIOs, which is not enough to activate all the 64 electrodes, which represent 512 GPIOs.
it can address 16 or 8 electrodes, for the 2- and 4-role configuration respectively.
We used gpio expander I2C (MCP23017). We have associated these components with an I2C multiplexer of type type TCA9548A from adafruit.
Given the reduced number of relays, the MUX board v2024 is interfaced with only two MCP23017 I/O expanders.
This combination allows to go up to 512 GPIOs and up to 128 electrodes. Each card has its own digital address between 0X70 and 0X77.
This means that up to 4 MUX boards v2024 (i.e. 32-electrode system) can be directly connected to a measurement board v2024.
In the following presentation for an OhmPi 64 electrodes, we will use the addresses 0X70 for channel A, 0X71 for channel B, 0X72 for channel M and 0X73 for channel N.
A newly introduced I2C extension board (featuring a TCA9548A multiplexer) allows to connect up to 32 MUX board v2024, equating to a 256-electrode system.
0X73 for the N channel. 4 MUX board will be needed to multiplex an OhmPi 64 electrodes.
In theory, up to 8 I2C extension boards can be connected to the measurement board, which would allow to pilot 2048 electrodes.
For obvious practical reasons, such a configuration couldn't be tested and is likely to be limited by the I2C bus being phisically too long,
which would prevent to reach so many GPIOs.
In an effort to mitigate supply shortages, a last addition concerns the power mosfet associated with the relays,
with the possibility to mount two types of components depending on market availability: either ZVN4206A or STP16NF06L.
Here, we will present how to assemble and configure a 32-electrode system, based on 4 MUX-board v2024 set up to address 2 roles / 16 electrodes each.
To build an ohmpi it is necessary to have 4 MUX boards, with 4 different addresses. It is therefore necessary to identify each board, by assigning an address, which will be allocated in the OhmPi code.
A MUX v2024 comes with 2 I/O expanders (MCP23017) addressing 16 relays each. They can be identified by the Raspberry Pi unit
We present here the addresses selected by default.
with their I2C address ranging 0x20 - 0x27. Two jumpers allow to shift the addresses two by two based on the following combination:
