Last night I got the three primary zone circuits instrumented and connected to the Arduino. I also have the two thermistors working but I have yet to run the outdoor sensor through the house wall. All this has been done with USB communication to the Arduino siting on the floor of the furnace room.<\/p>\n
It was therefore time to run a serial cable to my Linux file server one floor up from the furnace room. My recent experience with RS-232 over long runs pushed me to an RS-485 solution.So I pulled out the Sparkfun RS-485 boards I purchased a while back for the lightning detector. I had discovered a problem with their implementation. As delivered the boards are useless for multidrop communication, but will work just fine for a single point to point communication channel.<\/p>\n
I make up a cable harness and connected the CMOS logic to RS-485 converter to the RX\/TX pins on the Arduino, and I used the Arduino D13 logic signal to drive the RTS pin on the converter. I then went to upload a new sketch to the Arduino and it failed to load.<\/p>\n
At first I figured I had swapped RX and TX and therefore I was screwing up the bus, but a careful survey indicated I had connected it up properly. So I un-soldered the RX and TX pins and verified that I could upload. So next I connected the Arduino’s transmit pin to the converters receive. That worked as well. As soon as I did that I realized my problem.<\/p>\n
I have bus contention on the Arduino’s receive line. Both the USB UART and the RS-485 converter are trying to drive the Arduino RX line. This garbles the communication and prevents a good upload. So what to do?<\/p>\n
Well I could use one of the bit bang serial port libraries available to avoid the problem. But I really do not want to give up two GPIO pins when I already have two designed for serial communications. The easiest solution will be to put in a mechanical switch to isolate the RX\/TX lines during programming. That switch can be inserted into the cable harness and mounted inside the enclosure so it can’t be accidentally flipped.<\/p>\n
For a moment I wondered if I could program the Arduino over RS-485. But mu guess is no. The Arduino IDE does not likely support the mono directional RS-484 format. Oh well.<\/p>\n","protected":false},"excerpt":{"rendered":"
Last night I got the three primary zone circuits instrumented and connected to the Arduino. I also have the two thermistors working but I have yet to run the outdoor sensor through the house wall. All this has been done … Continue reading