SN65HVD72D -  RS485 - Page 1
Forum section: electronics / projects

author: jwhitmore
author_rank: Regular Contributor
timestamp: January 18, 2016, 07:14:37 pm
content: OK So this is probably going to be a ridiculous question but as much as anything I'm rubber ducking to see if typing will trigger something. So I want to talk to an RS485 based sensor from a uC. So I thought that if I got a chip that converts from the uC's UART output to RS485 it would be so simple. Wrong. I do think that it should be that simple but I'm struggling. I'm enabling the transmitter with the enable pin and I can see my four bytes of data from the UART but the RS485 out of the SN65HVD72D is just doing nothing.I've been over this a lot of times. I though maybe i had to put a 120R resistor across the 495 AB but that changed nothing. I'll try and attach a photo from my Rigol 1054I'm starting to think that I've got the device soldered the wrong way around there's a line at one end of the device so I assume that's marking pin 1.OK typing that has not helped :-( On the screen capture light blue at the bottom is transmit enable and yellow is serial. I'm sure nobody has probably used this device. I'll have to re-read the datasheet one more time but I could nearly perform it at this stage :-(John

author: nctnico
author_rank: Super Contributor
timestamp: January 18, 2016, 07:18:41 pm
content: There are two enable pins: one for receiving and one for transmitting. Perhaps you have these wrong?

author: jwhitmore
author_rank: Regular Contributor
timestamp: January 18, 2016, 07:52:47 pm
content: I've checked the select line. I'll attach the circuit diagram. I've left the two select lines on different IO pins as, according to the datasheet that will enable me to select Rx at the same time as Tx and get a loopback of what I'm transmitting. At present I'm not doing that as I'm not happy with the output at all.I might have to move to another solution, in terms of another chip but this is bugging me, and if I have made a mistake there's no doubt I'll make it again.Thanks for the thought.

author: nctnico
author_rank: Super Contributor
timestamp: January 18, 2016, 08:00:25 pm
content: When DE is high then everything on the D input should appear on the A/B lines. I'd start by checking the power supply and the logic levels of the inputs.

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 18, 2016, 08:02:57 pm
content: It's pretty difficult to screw up with an IC this simple so I have to ask if bought them from a reputable distributor or did you get 'em cheap off ebay or aliexpress or some other shady second line broker?

author: jwhitmore
author_rank: Regular Contributor
timestamp: January 18, 2016, 09:54:47 pm
content: Cheers MagicSmoker "difficult to screw up with an IC this simple" ;-) Loved that, and have to agree but somehow I'm living the dream. I bought from farnell in the uk so I have to assume it's good. I'll recheck all pins and see if I can't find what it is I'm missing.Thanks for all your help

author: ajb
author_rank: Super Contributor
timestamp: January 18, 2016, 10:13:17 pm
content: Just to cover things, since your schematic doesn't make this clear. You have the UART TX line connected to the driver input (pin 4), right? And you're driving the driver enable (pin 3) high?

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 18, 2016, 10:41:19 pm
content: Both an open and short circuit on the RS-485 bus will result in the R output always high, so that could be the problem.Otherwise, and assuming the basic stuff is covered - Vcc = 3.3V, !RE is low, DE is high, valid data at D input - the only other possibilities are a dead or miswired chip...

author: jwhitmore
author_rank: Regular Contributor
timestamp: January 19, 2016, 08:55:23 pm
content: School boy error! It was always going to be a school boy error. I thought I'd buzz'd the circuit out but I didn't do it right. There was a bad solder joint on the Tx select pin so although on the scope I was looking at a perfect select the device was seeing nothing. It was right I was wrong.Thank you all for your help and support. I have learned a little.