FX3U as Modbus Slave

[jimr007 0]

Hello, Has anyone setup an FX3U as a Modbus Slave and communication from a 3rd party device as the Modbus Master (i.e. PC) ? For hardware I am using an FX3U-32MT/DS with an FX3U-485ADP-MB on the left-side. Based on the "FX3U Series Communication Manual - MODBUS Edition" I have tried the following: 1. wrote only the logic to establish the FX3U as a Modbus Slave (see attached for communication detail). 2. wired based on the 4-wire diagram in the Mitsubishi manual: from the 485ADP to a 'B&B electronics USPTL4, (RS-485 / USB adapter), to a PC I used the 330 ohm dip switch setting on the 485ADP, and 330 ohm terminating resistors on the USPTL4. 3. Tried reading from the FX using several different widely available 'Modbus Master' programs running on a PC. With the following results: - On the 485ADP I see the RD light flash each time I try a read from the PC, but get a timeout error, (no response from PLC) - If it will help, I can post the various bits & registers in the PLC, but in summary; I see D101 (bus comm error counter) increment with every read from the PC. However, D8402 (Modbus Error code) is zero. Also I have tested with many variations of the above: Changing: baud rate, Slave Address, 2-wire and 4-wire, with and without terminating resistors, RTU / ASCII Tried all different Modbus read commands: (0x01, 0x02, 0x03, 0x04) Is there any other programming necessary on the FX3U to respond to requests from a Modbus Master? Any suggestions you can offer will be appreciated. Thank you, Jim Edited 1 Sep 2010 by jimr007

[kaare_t 155]

Maybe a silly question from me, but do you have the FX3U-CNV-BD card installed between the main unit and the ADP card?? Or do you have any other BD card installed?? Please let us know what you have between the main unit and the ADP module...

[jimr007 0]

Hello, Good news, I have successful communication between my PC and FX PLC. After some reading about troubleshooting RS-485 Modbus; I tried reversing the wiring from my RS-485 converter to the FX 485ADP module. Instead of wiring SDA to RDA, and SDB to RDB, I now have SDA to RDB, and SDB to RDA. This wiring change enabled successful communication. There must be a difference in polarity between my RS485 converter and the FX 485ADP. Thank you for your interest in helping out. Jim

[Inntele 60]

Jim, is normally to exchange entirely lines A and B with each other. About it constantly has been written on the forum. The connection of the transmission line A with the reception line B and vice versa, that's something new in the technique of connecting interface and a miracle that it works.

[skyfox 5]

I have been fighting with this for about a week as well. Jimr, If I understood you correctly, you changed the cabeling from your RS485 Adaptor to the PLC from; PC (Master) > FX3u-485ADP-MB Slave SDA > RDA SDB > RDB RDA < SDA RDB < SDB Changed to: SDA > RDB SDB > RDA RDA < SDB RDB < SDA What was your Temination setting at PLC Modbus Card end.... 110 Ohms or 330 Ohms? On yor RS485 to RS232 Adaptor Control.... RTS - On or off? SD - On or off? ECHO - On or Off? Also, this PLC (FX3u-80m) does not have the converter board that is shown in the modbus manual. Instead It has the USB board and then the Modbus card to the left of it. When I configured it for CH#1 comms, I got the error 201 stating the 485ADP was not detected. Then I configured it to CH#2 and the error went away. (yes I did look in the correct register for CH2, and the previous error on CH#1 went away after the power cycle). Right now, I can send data to it (RD on Modbus Card flashes) but the module never respond back (error code 204). I will try with the wires switched as jimr007 suggested instead of what the manual states the configuration to be SDA-RDA etc. This brings up another question, is this wiring switch over only good if we are using a PC as the master? What if another (Non Mitsubishi) Modbus RTU master is connected to this modbus card? Same apply? Thanks for providing us with feedback on how you corrected this. It's been a week and I poured over 4 or 5 different Mitsubishi Manuals, countless Modbus wiring diagrams. protocol specs etc. etc. w/o any luck (new to Modbus as most of my networks are either ethernet, DH+, or Controlnet type). Mitsubishi's Tech support has yet to return my call for help. I am sort of getting back in to Mitsubishi stuff again but stayed away from them for this very reason. But love the hardware when it works. Thanks again. Edited 8 Sep 2010 by skyfox

[kaare_t 155]

RS485 wiring has no specific standard (as RS232 has), so there's really no "exact" way to wire RS485... Different manufacturers draw different schemas for wiring, and they use different labels for the connections Therefore you'll just have to try to exchange all the wires until you find the correct way. When you can see that the LEDs are flashing as they are supposed to you have most likely found the correct cabling. However in your specific case, since you can see the RD LED of the card flashing it sound like you have the cabling correct as long as the SD LED is not lit up constantly....

[Inntele 60]

Dear Kaare, let me disagree with you. Yes, some manufacturers allow different designations lines of RS422/485 interface. It's bad but not so serious. However, I have never encountered devices in which the line to send and receive matched by name, but not identical to their destinations. If the author really has met with such product, the manufacturer of this product should be kicked out from the market by a filthy broom, because it takes away a specialist time, which, as known, money.