Nuts and Volts - November 2013

■ FIGURE 4. Dynamixel interface schematic.

with 1K between the Propeller pin and the pull-up will work, too (see the object code for the alternate connection).

■ FIGURE 5.

Dynamixel

After downloading the program, open a terminal window and then press a key. You'll see a menu for setting the baud rate for the driver — press [1] for 1,000,000 baud

scanner report.

(unless you know your actuators have been set to a different rate). After that, the code uses a method called scan to document any devices connected to the Propeller.

In the middle of the scan method, you'll find this loop which uses the driver's ping method:

■ FIGURE 6.

JonnyMac Jr. platform.

repeat id from $00 to $FD
  cursor_xy(12, 6)
  show_id(id)
 if (dy.ping(id) == id)
 cursor_xy(0, devices+8)
 term.str(string("--> "))
 show_id(id)
 devices += 1
 pause(50)

This code loops through all possible IDs, pings each, and if there is a successful response it is noted on the output. When complete, the number of devices detected is displayed. Figure 5 shows the result of the program running on my test setup.

Note that there is a 50 ms delay between packets. While talking with Kyle at Trossen Robotics, I got the impression that Dynamixels — even with their crazy-fast default baud rate — don't want to deal with more than about 30 packets per second. While I cannot find any documentation to support this, the guys at Trossen have a lot of experience with the Dynamixels and have been incredibly helpful when I've called. This helps explain the use of Sync Write; while a bit fussy to set up, it allows one to update registers in several actuators using a single packet.

While ping is useful to inventory devices connected to a system, that's about where its utility ends. To do useful things with the Dynamixel, we need to read and write registers. The driver includes low-level methods called read_data and write_data.