User Guide

Using the Servo Controller

The Pololu servo controller performs the processor-intensive task of simultaneously

generating 8 independent servo control signals. The servo controller can generate

pulses from 0.25 ms to 2.75 ms, which is greater than the range of most servos, and

which allows for a servo operating range of over 180 degrees.

Internally, the servo controller maintains a servo position value that is two times the

pulse width, measured in microseconds. Thus, the 1.5 ms neutral position, which is

1500 microseconds long, is represented internally as 3000. The internal values range

from 500 to 5500. Various interface modes allow the user to set the position value for

each servo in multiple ways, which are described below.

Serial Input

The serial commands sent to the servo controller

must be sent eight bits at a time, with no parity and

one stop bit (sometimes abbreviated 8N1). Logic-

level serial input must be non-inverted, meaning

that a zero is sent as a low voltage, and a one is sent

as a high voltage, as shown in the diagram to the

right. Any inverted serial input, whether at logic

levels or at RS-232 levels, may be connected to the

RS-232 serial input.

When you turn on power with your serial input connected, you should either see all of

the LEDs or just the yellow LED turn on. After the servo controller turns on and

determines the communication mode (see below), it waits for a serial input to

determine the baud rate. If the input line is low, it turns all LEDs on and waits for the

line to go high, which is the idle state for the serial line. Once the line is detected to be

high, only the yellow LED is turned on, and the servo controller waits for a serial input.

If the detected baud rate is too high, the red LED will turn on and the green LED will

flash quickly. If the serial rate is too slow, the red LED will turn on and the yellow LED

will flash. From this point on, the servo controller behavior depends on the

communication mode. Once you choose a baud rate, all subsequent transmissions

must be at that same baud rate.

Indicator LEDs

The green LED indicates serial activity: it should flicker whenever the servo controller

receives data. The yellow LED indicates a warning regarding position: either the

absolute or neutral position you have requested is out of range, or a combination of

neutral, range, and 7-bit or 8-bit position caused the internal position variable to go out

of range. The position will just be limited to the max or min, and the yellow LED will

go out when all requested positions are in range. The red LED indicates a fatal error

that prevents further operation (for example, a fatal error could be caused by invalid

serial input).

10011010

start bit stop bit

LSB MSB

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