Servo - September 2007

FIGURE 1. Upon
power-up, nodeB sends
a status message telling
nodeA that it has
turned off the nodeB
LED. The CAN receive
code running on nodeA
adds the bottom
two lines of status
information to the
original menu display.

FIGURE 2. Looks like nodeB doesn’t have much to say. However, this simple message says it all.

That puts nodeA at the advantage when both nodes attempt to transmit at the same time. That’s nice to know, but the CAN bus won’t be that busy in this simple CAN application.

Initially, we’re only going to control and monitor the status of the LED on nodeB via nodeA. So, we must also define the LED control commands that will be embedded within the CAN message:

#define led_off 0xA0
#define led_on  0xA1

We now have definitions for all of the elements that we must include in our CAN messages. All that is left to do is establish the byte ordering within a transmitted CAN message. Here’s how our CAN messages will be formatted:

CAN ID = sending node’s CAN ID
data[0] = logical address of receiving CAN node
data[1] = LED command

from the viewpoint of nodeA. Interaction with nodeA is done via nodeA’s 57600 bps RS-232 port and Tera Term Pro. Tera Term Pro is a personal computer terminal emulator that you can get as a free download on the Internet. In fact, both of our CAN nodes are fitted with 57600 bps RS-232 ports. That allows us to use the same RS-232 driver code on both nodeA and nodeB. The RS-232 driver firmware on both of our CAN nodes is interrupt-driven. Our application uses the C printf function, which does not utilize the CAN node’s transmit interrupt handler code. Thus, the interrupt activity on the pair of CAN nodes’ RS-232 ports is only invoked by RS-232 receive activity. To check for an incoming RS-232 character, we simply call the CharInQueue function. If there are any characters in the CAN node’s RS-232 buffer, the CharInQueue function will return a logical TRUE. Retrieving a character from the RS-232 receive buffer is accomplished by this simple line of code:

bytein = recvchar();

If we turn all of our definitions into code, we end up with a transmission sequence that looks like this:

dataLen = 0;
id = myid;
data[0] = nodeB;
++dataLen;
data[1] = led_on;
++dataLen;
while(!ECANSendMessage(id, data, dataLen,
                     ECAN_TX_STD_FRAME));

The transmission sequence code I have presented here is sending a message to nodeB telling it to turn its on-board LED on. Note that the dataLen variable value is calculated as we add data elements to the message.

The standardization of our CAN message transmissions also forms the foundation for our CAN message receive parsing sequence. Our CAN receive message firmware will always look for the logical CAN node address in the data[0] position. If the receiving CAN node sees that the incoming CAN node address is its own, it then parses the data[1] memory slot for the LED command. Nothing to it.

Application in a CAN

Let’s look at the nodeA and nodeB CAN applications

56 SERVO 09.2007

Both of our CAN nodes are also equipped with a firmware-implemented real-time clock. The clock is fed by the PIC’s Timer3 and is interrupt-driven. The real-time clock is behind the 1 Hz flash rate of the nodeA LED. The nodeB CAN node has the same real-time clock capability. However, we want to control the LED on nodeB exclusively. To gain total control of the nodeB LED, we must disable the nodeB real-time clock timer (Timer3) or simply comment out the LED toggle statement in the Timer 3 interrupt handler. Just in case we need millisecond timing windows, both of our CAN nodes also run their Timer2 clocks on a one millisecond interrupt interval.

The very first task that nodeA executes is to clear the Tera Term Pro terminal window. Note that in the code I have provided for you that we are using VT-100 commands to place our characters in the Tera Term Pro terminal window. Using VT-100 screen formatting commands makes easy work of putting together a nice looking menu. To clear the Tera Term Pro window, we simply place a call to the cls function, which consists of this line of code: