Electronic – Two MAX3140 (SPI to RS485) connected to one another

I found an interesting read from Texas Instruments here called Extending the SPI bus for long-distance communication by Thomas Kugelstadt. It sounds pretty difficult to just make it work keeping it as SPI, but they do say you can get (up to) 100m+.

I would say, definitely do the SPI-to-RS485 conversion. Use the differential drivers through shielded twisted pair cables to get that 100m distance. This is the sort of task it was designed for.

Maybe even use fiber optics for the RS485 if you want minimal noise or delay problems.

The capacitance of 100m of cable will ruin any attempt at synchronous (clock based) high speed serial protocols like SPI, and making the voltage 12V will make the problem even worse, as the slew rate of the transmitting op-amp will need to be much higher than if it was only driving 3.3 or 5V over the same capacitance, because to change logic state it has to produce more change in voltage on the line.

Differential signals used in RS485 can operate at a nice low level (+-1.5V usually) and still be very fast. RS485 can usually do the standard baud rates up to 4000ft of cable. Because 100m is only around 300ft, you can expect the standard 115200 baud rate to work fine, or maybe even try the higher baud rates if your chips will let you, to squeeze as much performance out of your SPI->RS485 link as possible.

After the RS485 transmission, your LED driver chip can then receive SPI protocol commands from the protocol conversion chip, as if there was nothing betweeen the LED chip and the sender.

There WILL be delays for the SPI-> RS485 -> RS485->SPI conversion (see the datasheets of the protocol conversion chips for these numbers) and of course RS485 has a slower overall bandwidth than SPI. There is an NXP chip "SC16IS752" I found which can do this SPI to RS485 for you, otherwise you could look at doing SPI to UART, then attach a RS485 transceiver ship on the output of the UART before going out onto the cables.

You can certainly do this within limitations. The main thing you have to be concerned with is the voltage drop that will occur along the wires. One reason that high voltages such as 48V are used is that the drop along the cables is a smaller percentage of the total voltage than say if 9V or 12V were passed down the wire. Put another way, you can pass more power down the cable at 48V than you can at 9V or 12V when living with a given cable voltage drop.

Now with RS485, even though it is a differential bus, the receiver parts used will have a maximum common mode voltage range specification that must be obeyed in order to ensure reliable operation without sacrificing silicon health. The voltage drop in the cables due to the power delivery scheme have a direct impact on the common mode voltage that the receiver will see with respect to its RS485-A and RS485-B differential pair pins.

Also be aware that RS485 modes are not particularly low power when you consider that the driver must be able to drive the terminations on the bus. Typically there are required terminations of 120 ohms between the differential pair at each end of the bus. This means that the drivers are sourcing and sinking current across a net 60 ohms when the bus is active. If your driver is pushing out 5V signal levels this corresponds to a current of ~83mA which is probably 5 to 10 times more than the typical low power microcontroller node current consumption.