Talking to 3 595's via Max/MSP

This is a follow up to a previous post regarding the connection of three 595 registers to an Arduino board and is a response to a question someone has asked me. What i did not cover in that post is how to actually make the connection between a host computer and the shift registers.

Let us review quickly how the Arduino sketch passes on the data to the 595 registers. Basically, the Arduino waits until its serial buffer has received three bytes. Then, each of these bytes is read and shifted out into the registers. The first byte will go to the 595 IC that is furthest away from the Arduino in the serial input / Q7' output chain (see the previous post for the hardware setup). The baud rate is 57600.

The 8 bits from each byte represents each of the outputs of the 595 register, like so:

bit 7 = 595 output Q7 (pin 7)
bit 6 = 595 output Q6 (pin 6)
bit 5 = 595 output Q5 (pin 5)
bit 4 = 595 output Q4 (pin 4)
bit 3 = 595 output Q3 (pin 3)
bit 2 = 595 output Q2 (pin 2)
bit 1 = 595 output Q1 (pin 1)
bit 0 = 595 output Q0 (pin 15)


So, it is possible to use an 8 bit value (from 0 to 255) to independently represent the 8 digital output pins Q0 to Q7 on the 595. It follows that we can control the three 595 shift registers quite easily from Max/MSP with a patch such as this one:

The main patch.

The sto.8b1b encapsulation that is referenced in the main patch.

How does this patch work? Well, twenty-four toggles each represents a bit in three bytes of information. Whenever the value of a toggle is changed, then the entire updated byte of information is sent out of the bottom of the sto.8b1b encapsulation as an integer (0 to 255).

These three bytes need to be sent for every change that occurs within the three bytes because the Arduino is expecting to see three bytes of serial data in order to set the three shift registers. Furthermore, these three bytes must be sent in a particular order - the third shift register must be sent first, the first shift register must be sent last.

In order to get the proper sequence of bytes, the three values are grouped together in a list via the pack object. The list in then unpacked, and because of Max's right-to-left order, the third item in the list is sent to the serial object first and so on.

The serial object is connected to the Arduino at a baud rate of 57600.

It is evident that this may be an inefficient means of updating the three shift registers, because three bytes must be sent in order to change a single bit of information output. This may suffice in many situations. But it is possible to use a numbering prefix (1 to 3) to control the register. This would require a reworking of the Arduino code.

Download this patch here: http://milkcrate.com.au/_other/downloads/max_patches/Three_595s/.