Modbus implementation

scrwld · Joined: 28 Jun 2010 Posts: 3

Hello everyone. I'm new to this forum.

I have the same problem, works with max232 and MAX485 does not work, take an inverter gate I had on hand (74HC14) and now works, no 100 percent.From my experience with modbus have never had to add some additional device to work.

continue to seek the reason for this when have some results I will indicate.

regards

luis lopez

http://www.subirimagenes.com/otros-pic16f877232485-4728280.html

yerpa · Joined: 19 Feb 2004 Posts: 58 Location: Wisconsin

You could try swapping your two serial data lines (data+ and data-). This should perform an inversion just like your inverter chip, without requiring the inverter chip.

SimpleAsPossible · Joined: 19 Jun 2004 Posts: 21

It looks like the issues here MIGHT have to do with the software enabling the transmitter, so a change to the hardware might help.

I have successfully used a circuit I found in a Circuit Cellar RS-485 article a few years ago. It uses a 555 timer as a retriggerable one-shot, triggered by the negative edge of TX. The output is used as the enable signal for the 485 driver. Once TX goes high and stays high, the one-shot times out, after approximately one character time. The only connections to the driver circuit are the RX and TX lines -- no enable necessary.

If it turns out that the problem really is that the software is not correctly handling the enable line, this will take care of it. Or at least diagnose it so you can find a solution you like.

scrwld · Joined: 28 Jun 2010 Posts: 3

yerpa, simpleaspossible thanks for your response.

I change the connection and runs but tends to give errors and lost communication. The correct connection is A-A B-B.

The program has a break of about 3 milliseconds and this is the best value to give me a result, the problem is that after transmission during that time the slave has ability exit and this causes collisions with other slave or master.

iman_zahari · Joined: 20 Jul 2010 Posts: 15

Hey guys, in ex_modbus_slave.c , what does these means?

int8 coils = 0b00000101;
int8 inputs = 0b00001001;
int16 hold_regs[] = {0x8800,0x7700,0x6600,0x5500,0x4400,0x3300,0x2200,0x1100};
int16 input_regs[] = {0x1100,0x2200,0x3300,0x4400,0x5500,0x6600,0x7700,0x8800};
int16 event_count = 0;

Does this really implies that the pin b0 and b2 will always be high? If that is so, why is the inputs are already defined as b0 and b3 will always be high, meanwhile b2 will be low? This will cancel of the coils output at b2?

The other thing is, why are the holding registers and input registers are set at 0x8800, 0x7700 etc? Why it doesn't use as only 0x0000, 0x1000 etc?

Thanks.

iman_zahari · Joined: 20 Jul 2010 Posts: 15

I tried using modbusview. When I tried to force coil 01 to go high, the reply is high,

transmit:
[01] [05] [00] [FF] [00] [8C] [3A]

receive:
[01] [05] [00] [FF] [00] [8C] [3A]

So I guess it is correct. But, when I tried to read the value of the coil at 01, the result is like this,

transmit:
[01] [01] [00] [00] [00] [64] [3D] [E1]

receive:
[01] [81] 02] [C1] [91]

Which is an exception value of illegal data address. I am not very sure of what it means, does this mean that I have to write an exact address for this type of coding in the ex_modbus_slave.c file?

I am using the exact code with modbusview set to master with USE_WITH_PC 1. Nothing else changed. Can someone help me?

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

The data you show are different from the addresses and quantities used in the MODBUS-Master example. The quantity of 100 used in read_coils request is outside the range provided by the slave example. It's correct to throw an execption in this case, check the code.

iman_zahari · Joined: 20 Jul 2010 Posts: 15

The length is the buffer size rite? Sorry but I'm not sure where to locate it.

iman_zahari · Joined: 20 Jul 2010 Posts: 15

All right. I got it. It is 5. I got confused by the buffer size. But if int8 coils = 0b00000101 is the number of length, then what int8 inputs = 0b00001001 is supposed to mean?

iman_zahari · Joined: 20 Jul 2010 Posts: 15

oo ok. i got it all now. tqvm FvM

bbnnbb · Joined: 09 Mar 2012 Posts: 3

PLEASE SIR I HAVE THE SAME PROJECT AS YOU CAN YOU HELP AND ALSO TEL ME WICH HMI YOU USED;
BEST REGARDS,
_________________
HELLO EVERY BODY!,AM WORKING IN MY FINAL PROJECT IN IMPLEMENTATION OF THE MODBUS AND INTERFACING WITH HMI SO I NEED HELP IN MY PROJECT (I USE LIBRARY MODBUS OF CCS)

bbnnbb · Joined: 09 Mar 2012 Posts: 3

please sir can you advise me with which HMI you choose cause i did the same work as you except the analog output (not yet problems with MCP4921) and now i need an HMI software.
_________________
HELLO EVERY BODY!,AM WORKING IN MY FINAL PROJECT IN IMPLEMENTATION OF THE MODBUS AND INTERFACING WITH HMI SO I NEED HELP IN MY PROJECT (I USE LIBRARY MODBUS OF CCS)