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BeagleBone Black STEP/DIR Interface (BBB_SDI)
See the image below for STEP/DIR and I/O interface pinout.

STEP/DIR Interface
The STEP/DIR outputs are inverted on the board to allow for active LOW signals using standard active high signals from the BBB.
These can be used for single axis drives that have optically isolated inputs.

IDC26 Header Interface
The IDC26 Header has the following pinout:
Pin	Port A	
1	8-7		I/O
3	9-25		I/O
5	9-27		I/O
7	9-29		I/O
9	9-31		I/O
11	9-33		I (Analog)
13	9-37		I (Analog)
15	9-39		I (Analog)
17	9-16		I/O
19	9-24		I/O
21	9-26		I/O
23	9-28		I/O
25	9-30		I/O
2	9-36		I (Analog)
4	9-38		I (Analog)
6	9-40		I (Analog)
8	9-42		I/O

IDC26 Port Pins 10, 12, 14, 16, 18, 20, 22, 24 are attached to GND.
IDC26 Pin 26 is attached to 5 Volts.
IDC26 Pin 1 is an output and this signal is also used to enable the on board buffers for the IC26 interface, the Screw terminal interface and the STEP/DIR interface. It needs to be programmed active low to enable the buffers. It is BBB P8-7.
The other IDC26 signal pins are buffered through FET switches are inputs or outputs as based on the direction setting inside the BBB except those labeled (Analog) which are inputs with a maximum voltage of 1.8 Volts. Non Analog signals are 3.3 volts rated.

Screw Terminal Interface
The Green Screw terminal block has dedicated I/O which are buffered through an LVC541 chip, and are as follows:
P8-9, P8-10, P8-14, P8-18 and P9-41 are inputs are are RC filtered.
P8-13, P8-19 and P9-14 are outputs.
The inputs on the screw terminal are +5V tolerant, while the outputs use a 3.3V signal voltage.

In the example in the picture below, the +5V (red and green wires), the X-STEP(brown wire) and X-DIR(blue wire) signals are routed from the BBB_SDI to the single axis drive using simple wires (keywords) [Arduino jumper wires Male to Female] available on ebay.
Drives like the one shown are active high enabled, so to run the ENA- output can be left floating. The ENA- can be routed to a STOP switch that pulls the ENA- LOW (GND) while having +5V attached to the ENA+ input, both available on the Screw Terminal block.

BBB   Function
----- ----------
P8-12 X-STEP
P8-11 X-DIR
P8-16 Y-STEP
P8-15 Y-DIR
P9-15 Z-STEP
P9-23 Z-DIR
P9-11 A-STEP
P9-13 A-DIR

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