Using fgetc() not working?

nmeyer · Joined: 09 Jul 2004 Posts: 70

I have a design that uses a PIC18F8722 and i use both uarts on this part. The first one i have as serial out to a PC. The second i have driving a RS485 converter chip to talk on a RS485 bus. I am able to write to the bus with my chip just fine. I can send data and get the devices i am talking to working. However, i also need to read data back from the devices and this is where i am having issues. I have tried to use the fgetc() and kbhit() to read in the data values. However, what i read in is garbage, i get numbers but they are not what i expect. I have attached a rs485 monitor to the rs485 bus and i can see the data i am writing and i can see the data that the devices is writing back. so , know the devices is sending back valid data, i am just not reading it in correctly. I have also tried to use #int_rda2 with no better results. Here is some code that i am testing with. This was my full program, but i have hacked it back and there fore there are some things that are not used. Version 4.062

Humberto · Posted: Mon Apr 28, 2008 12:02 pm

Just some basical comments after see your code at first glance:

1) To transmit in RS485 mode, you need to toggle the transceiver control lines. In your code
I do not see anthing doing this. See the ENABLE option.
2) Inside the INT_RDA interrupt handler, it is not necesary to use the function kbhit()
because you already are in the interrupt vector. If the interrupt was triggered is
because the UART detected the Start bit, hence kbhit() is redundant.
3) Your code inside the interrupt handler is quite twisted. You can get the same using a
storing buffer and then doing a parser inside the main().
4) It is not a good practice to use several getc() inside the same function.

Humberto

nmeyer · Joined: 09 Jul 2004 Posts: 70

Thanks. I am toggling an enable pin on the rs485 manually. I saw the enable in the #use set up and i am going to implement that. Yes, the int is rather poorly written, but i was trying that vs just reading with the kbhit() function. I am going to work on cleaning this all up and reducing it to just a read and write program to make sure i do not have anything else that is conflicting.

nmeyer · Joined: 09 Jul 2004 Posts: 70

I have been able to get it to work better, but now it is not consistant. Some times when i power up my unit it runs for minutes and writes and reads the correct data with no glitches. then some times it will read errounous values all the time. I have attached my updated code, this is the best i have seen so far.

Ttelmah · Guest

As a comment, without looking at the code, how is the RS485 bus terminated?.
On RS485, you have to either use receivers, that 'see' an undriven bus as 'inactive, and return a logic high to chips reading the bus, or you have to ensure that the lines on the bus have a small 'bias', so that an idle bus is read as the inactive state. The normal way, is to terminate the bus, with a resistor to +5v, then a resistor between the lines, and then another resistor to ground, whose total parallel impedance, gives the required termination impedance, and also applies the slight bias needed. Typically 1.2KR to +5v, from the 'A' line, 120R between the lines, and then 1..2K to ground, from the 'B' line, gives a good match to 100R cable, and provides the required bias. Some manufacturers provide terminator chips, that give this bias, and some do line receivers guaranteed not to need this. Unless you have this, you _will_ be seeing huge amounts of pretty random data when the bus is not driven, and this could easily be making the code become erratic...

Best Wishes

nmeyer · Joined: 09 Jul 2004 Posts: 70

I do have a 120 ohm between A and B terminating the line, but i do not have any pull up or pull down resistors.