Pololu Jrk USB - Manual (Page 38)

Connections
Connect the TX line of the master device to the RX lines of all of the slave devices. Commands sent by the master
will then be received by all slaves.
Receiving serial responses from one the slave devices on the PC can be achieved by connecting the TX line of that
slave device to the RX line of the jrk.
Receiving serial responses from multiple slave devices is more complicated. Each device should only transmit when
requested, so if each device is addressed separately, multiple devices will not transmit simultaneously. However, the
TX outputs are driven high when not sending data, so they cannot simply be wired together. Instead, you can use
an AND gate, as shown in the diagram, to combine the signals. Note that in many cases receiving responses is not
necessary, and the TX lines can be left unconnected.
If there are multiple slave jrks, you can connect all of the jrks’ ERR lines to a single input on a microcontroller to
monitor the error state.
Whenever connecting devices, remember to wire the grounds together, and ensure that each device
is properly powered. Unpowered devices with a TTL serial port can turn on or partially on, drawing
power from the serial line, which means that extra care must be taken when turning power off and on
to reset the devices.
Sending commands
The Pololu Protocol should be used when multiple Pololu devices are receiving the same serial data. This allows the
devices to be individually addressed, and it allows responses to be sent without collisions.
If the devices are configured to detect the baud rate, then when you issue your first Pololu Protocol command, the
devices can automatically detect the baud from the initial 0xAA byte.
Some older Pololu devices use 0x80 as an initial command byte. If you want to chain these together with devices
expecting 0xAA, you should first transmit the byte 0x80 so that these devices can automatically detect the baud rate,
and only then should you send the byte 0xAA so that the jrk can detect the baud rate. Once all devices have detected
the baud rate, Pololu devices that expect a leading command byte of 0x80 will ignore command packets that start with
0xAA, and jrks will ignore command packets that start with 0x80.
4.i. Serial Example Code
4.i.1. Cross-platform C
The example C code below works on Windows, Linux, and Mac OS X 10.7 or later. It demonstrates how to set the
target of a jrk by sending a Set Target command to its Command Port, and how to read variables from the jrk. The
jrk’s input mode must be set to “Serial” and the serial mode must be “USB Dual Port” for this code to work. You will
also need to modify the line that specifies the name of the COM port device.
This code will work in Windows if compiled with MinGW, but it does not work with the Microsoft C
compiler. For Windows-specific example code that works with either compiler, see
Section 4.i.2 .
// Uses POSIX functions to send and receive data from a jrk.
// NOTE: The jrk's input mode must be "Serial".
// NOTE: The jrk's serial mode must be set to "USB Dual Port".
// NOTE: You must change the 'const char * device' line below.
Pololu Jrk USB Motor Controller User's Guide © 2001–2014 Pololu Corporation
4. Using the Serial Interface Page 38 of 45