Laptop link (FT232)

The laptop reaches the servo bus through an FT232R USB-serial adapter (USB ID 0403:6001, serial B00033ZX) on the base pins 13/14/GND. On macOS it is /dev/tty.usbserial-B00033ZX, with Apple’s built-in AppleUSBFTDI driver.

The latency timer

The FT232R keeps received bytes until it has 62 of them, or until its latency timer expires. The default is 16 ms. Every reply from the robot is short, so every reply waited 16 ms. This was most of the “20 ms get_angles” problem.

Command	Latency timer 16 ms	Latency timer 1 ms
Stock get_angles	20.5 ms	8.3–9.0 ms
Stock is_power_on	16.0 ms	2.0 ms
Direct SYNC READ of 6 servos	—	~2.0 ms

Apple’s driver has no setting for the timer. The chip accepts the FTDI vendor request on endpoint 0 while the port is open:

import MyCobot
MyCobot.set_latency_timer(1)     # selects the FT232R by serial number
MyCobot.get_latency_timer()      # 1

or python tools/python/ftdi_latency.py 1.

Caution

The timer resets to 16 ms when the adapter is unplugged or the robot is power-cycled. Set it again each time you connect. The scripts do this.

On Linux, write 1 to /sys/bus/usb-serial/devices/ttyUSB0/latency_timer.

The laptop loop

MyCobot.play_trajectory (Julia) and tools/python/play_trajectory.py do this in each cycle:

1. Interpolate the commanded joint angles at the current time.
2. SYNC WRITE the goal positions of all six servos.
3. Wait 1 ms (Julia) or drain the port (Python). See rules for writes.
4. SYNC READ position, speed and load of all six servos.
5. Stop and hold if a joint is more than 20° from the plan.

Player	Loop rate	Notes
Julia	~285–300 Hz	Occasional 30–80 ms stalls. The loop allocates 7.3 KB per cycle, so the garbage collector runs.
Python	~236–250 Hz	flush() drains the port, which costs ~1 ms per cycle.

The Julia player can cap its rate (rate= keyword, --rate= in play_plan.jl).