6 extra grey wires (TP20-TP25) 2.7 build add-ons



What kind of hardware could I connect to the extra 6 grey wires? Servos? What is the communication protocol? I2C?



Hi Andrew:

The six extra wires are just that, wires. They lead to test points TP20-TP25 on the controller board, which are not hooked to anything. You can put any kind of signal you want over those wires, you just have to internally route the signal to the appropriate test point on the controller board.

Here at OROV the most popular use for those wires so far has been to send power from the two spare PWM channels (PWM3 and PWM4, I think) out to the outside world, where they can power external lights. If you wanted to put a gripper arm on the ROV, you could run signals to a servo over those wires. They're free for you to do whatever you want with them.



Thanks a lot Walt. That was very helpful.

As a follow-up...say I wanted to run many servos, could I connect this thing to it? If so, Do you (or anyone else) know how it should connect to the control board?




I think I answered my own question...good guide at adafruit.com...


There are +5v, SCL, SDA, and GND terminals on the 2.7 control board, next to the power terminals for the lights and lasers. I would assume that the +5v, SCL, SDA, and GND terminals would be connected to the matching terminals on the adafruit PCA9685 16-channel-pwm-servo-driver? Can anyone confirm?

Communication protocol is I2C, correct?



Hi Andy:

There are many, many ways to hook up multiple servos to the OpenROV platform. Using a board like the one from Adafruit is a good choice if you don't want to worry about how the timers are assigned on the OROV controller board's embedded Arduino Mega. It's not needed if you're just adding a couple of servos, but if you want to add a lot, its a good way to go.

If you are keeping the Adafruit board inside the pressure vessel and running the servo wires out through the six auxiliary wires, then the wiring you mention above is correct.

If you are going to put the Adafruit board outside the pressure vessel, then there's an easier way to do it. From what I can tell, the board uses different power supplies for the digital interface and the servo driving. So you can run the interface off of the 3.3V I2C bus that already goes outside of the pressure hull (to the depth sensor and compass), and then just run a +5V wire and a ground wire to power the servos themselves. So you should only need to use two of the auxiliary wires to put 16 servos outside the ROV.

If you haven't done much electronics before this may take a bit of head scratching before it all sinks in. I'd start by reading over the Adafruit documentation a number of times.



You're the man! Thanks Walt.



I’m looking at adding 2 additional thrusters to the OpenROV design. In order to do that I will need to have 6 wires (2x 3 servo control wires) exit the main ROV housing to control the ESC’s. If I add the IMU sensor, will that use any of these 6 free wires? If not, can I use those wires for my intended purpose? If so, then I will likely have to run 6 more wires (and maybe use a DB9 connector) and run it through the opposite end cap.



The 4 wires required to run the IMU and depth sensor are already in the endcap. They don’t use any of the 6 spares.



Outstanding. So I can use those for the 2x servo controls for the additional thrusters. This is starting to look very do-able.



I do have one comment about using the six spare wires to run two additional thrusters:

The PC board traces between the test pads TP21-25 and pins 21-25 on the DB-25 are not sized to handle the current of a motor- they were meant for instrumentation, lights, and stuff like that. The traces were beefed up a bit when the 2.7 revision of the board came out, but they are still not really suitable for motors. So if you’re going to mount the motors on the board (say, piggybacking them over top of the existing ESCs), and are going to mount the ESC outputs into the holes for TP21-25, I would add some additional wires on the bottom of the board, running between TP21-25 and pins 21-25 of the DB-25 connector. These wires, in parallel with the traces already on the PC board, will help share the load, and keep the traces on the PC board from melting.



Thanks @Walt_Holm, I suspected that the boards might not be up to handling 30 amps so was planning on running the servo wires out of the main cylinder and connecting them to Blue Robotics integral ESC’s (the ones that are part of the thruster). If I go this route I may have a separate housing just for the high amp thruster wires and connections.