+ User Manual
I2C and Emergency Stop
The + has A new connector that was not on the older boards: J22. It can be seen
on the cover photo, next to the two push buttons. This connector is divided in two parts
and accepts either one single 6-pin mating part, or two separate connectors, one 4-pin part
for the I2C bus and one 2-pin part for the emergency stop signal.
Emergency Stop Input
The ESTOP input can be used to disable all outputs in case of an emergency situation. The
active emergency stop state is indicated by a red LED (D24) close to the ESTOP connector. As
long as the ESTOP state is asserted, the I/O-enable signal on the board will be suppressed in
hardware. No software is involved.
By default, the emergency stop signal is level sensitive. If jumper JP2 on the back side of the
board is closed, the signal becomes edge sensitive. A short activation of the ESTOP state will
then be latched to remain active until the board's I/O-enable signal is negated briefly via
software.
The ESTOP signal should connect to a normally closed contact or some open collector / open
drain type sensor. When not used, a jumper
or piece of wire can replace the sensor. An
external opto coupler / isolator can be used to interface to 24 Volt signals.
A normally closed contact should pull the ESTOP input low (to GND) during normal operation.
Breaking the contact will activate the emergency stop state. An opto-coupler with open
collector output can be used to interface with industrial level (e.g. 24 V) signals.
This way the board can be used without when the emergency stop is not required, or during
set-up and testing.
I2C Bus
The BeagleBone's I2C bus that also connects to the cape's EEPROM and AD-converter, is
connected to the first four pins of J22 (J22A).
A bi-directional level shifter converts the BeagleBone's 3.3 Volt bus to the levels used on the
external bus and isolates the bus if the external device is powered down
. The supply voltage
used by the external device(s) has to be connected to the I2C connector as reference. Any
voltage in the range 3.3 up to 5.0 Volts can be used for the external devices.
An external device requiring only a several tens of milli-Amperes at 5.0 Volts could take this
supply from one of the limit sensor supply pins. This way no additional external supply is
needed and in many situations not all inputs will be used leaving a connector with supply pin
free to use.
† The + board comes with a red jumper installed on J22B.
‡ If this wasn't done, the I2C bus would hang and the BeagleBone wouldn't be able to detect the cape properly
once the external device has no power.
version 1.4.9 (09/04/14)
page 36
