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stoyanoff
Joined: 20 Jul 2011
Posts: 375
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| Problem with RS485 |
 Posted: Wed Jul 15, 2015 7:58 am |
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Greetings! I have a device with RS485 output and a PCB. My PCB has PIC18F86J65 controller and ADM3485ARZ. I have 120ohms terminator. The problem is I can't receive anything from the device. I have 4.7k resistors on A and B connected to Vcc and GRD through jumpers. I've tried a lot of combination. The device doesn't respond! Is it a problem my RS485 chis in on 3.3V?!
I'm sure my hardware is working because I tested it with another PCB.
Please, help?! |
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asmboy
Joined: 20 Nov 2007
Posts: 2128
Location: albany ny
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| Posted: Wed Jul 15, 2015 8:10 am |
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Your post is a bit unclear.
You say:
| Quote: | I can't recieve anything from the device
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then say | Quote: |
sure my hardware is working because I tested it with another PCB. |
SO you CAN receive something with another bit of hardware or not?
got CODE and a schematic of where you are at ??
And a clearer explanation of what is working for you and what is not.
if you try to send data in a tight loop can you see signal on the RS485 output with an oscillioscope ?? or not |
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ezflyr
Joined: 25 Oct 2010
Posts: 1019
Location: Tewksbury, MA
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| Posted: Wed Jul 15, 2015 8:48 am |
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Hi,
How are you testing this? The ADM3485ARZ seems to be a 3.3V version of the ubiquitous 8 pin RS485 driver IC such as SN75176B and MAX485. So, assuming that your wiring is correct, and that you are *properly* handling the chip enable for each device, this should be an easy task. As was noted, it's hard to say anything specific until we see code and a schematic!
When I was doing RS485 development, I bought a USB-to-RS485 converter for my PC. Using the converter and a terminal program allowed me to work on one RS-485 node at a time to ensure things were working correctly. Only then did I connect multiple nodes together! These adapters can be purchased at places like B&B Electronics, and Sparkfun.
So, post the following if you want more help:
1. Code
2. Schematic
3. Vdd voltage if it's not obvious on the schematic
4. Compiler version
_________________
John
If it's worth doing, it's worth doing in real hardware! |
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guy
Joined: 21 Oct 2005
Posts: 297
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| Posted: Sat Jul 18, 2015 9:38 am |
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| Maybe the 3.3V with the 120 ohm termination doesn't create enough voltage difference for the receiving edge. Try a higher value termination resistor or remove it altogether for testing. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19451
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| Posted: Sat Jul 18, 2015 2:00 pm |
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No, RS455, is RS485. 100 to 120R termination. The drivers warrant to give over 2v into 50R.
ezflyr comments are spot on, as is asmboy's request for a schematic and code. |
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drolleman
Joined: 03 Feb 2011
Posts: 116
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| Posted: Sat Jul 18, 2015 10:35 pm |
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you are missing the pull up and pull down resistors. look at this site he is very good on 485
http://www.rs485.com/ |
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