Eight slave problem with sn75176 - Page 1
Forum section: electronics / beginners

author: Karamel
author_rank: Regular Contributor
timestamp: September 25, 2016, 01:45:13 am
content: Hi everyone,I have got a question at the moment. Today, i was trying multiple Rs485 connection and menegament them. I set ten very basic circuits which had just microcontrollers, 75176 transceivers also some resistors and capacitor which they need.So, i got a successfull connection with seven device but, when i connect 8th device, it doesnt metter whichone, connection was losting. I am on the road now, i can not therefore publish my schematic of basic circuit but tomorrow, i will. Actually, i had used recomended circuit in datasheet.Anyway,Is there anyone who had dealed with a problem which is like that?Best regards,KaramelP.s: By the way, i tried all combination for all. I think, there was no any problem of transceivers or etc. I am thinking that line had a problem. However, i read that sn75176 can set up to 32 devices?

author: rstofer
author_rank: Super Contributor
timestamp: September 25, 2016, 04:21:07 pm
content: The terminating resistors need to be at the physical end of line. You're right, the drivers should support 32 devices and it's a current limitation, not a topology problem.There is also a minimum node spacing based on the distributed capacitance of the PCB traces and the cable:http://www.ti.com/lit/an/slla272b/slla272b.pdfHere is a neat multi-master RS485 protocol. I don't know if you need this but I have found it helpful in a project I did some years back.http://www.bdmicro.com/code/robin/

author: Karamel
author_rank: Regular Contributor
timestamp: September 27, 2016, 08:16:36 am
content: Hi,Thanks forreplying. I, actually, used below circuit. (think that, s3 and s4 short circuit) and one's has a 120 ohm therminal resistor. (master transmitter)But i said that it doesn't work any eight device.P.s: my pcb design is compatible with differential pair wire and they less then 10mm so, i think, there is no any problem?

author: rstofer
author_rank: Super Contributor
timestamp: September 27, 2016, 02:11:40 pm
content: It appears that R3 is present on every node if I am understanding your circuit. This would result in a bus loading of 120 ohms divided by 8 or about 15 ohms. That would take a lot of power from the driver. Make sure you have this resistor only in the first and last device.You can close SW3 and SW4 on one device to center up the signal voltages. But only on one device. Otherwise, you have 550 Ohms divided by 8 or about 70 Ohms pulling the A signal up and the B signal down. With the termination resistors in place as mentioned above, there is no way the driver can handle the current.Read through those documents I linked. I'm sure they will help.Have you looked at the signals on a scope as you add more nodes? Are the voltage levels declining as more devices are added?If you mean your bus signals are only 10 mm long, I don't think this meets the Minimum Bus Length shown on Page 6 here:http://www.ti.com/lit/an/slla272b/slla272b.pdf

author: Karamel
author_rank: Regular Contributor
timestamp: September 28, 2016, 03:46:46 pm
content: You misunderstood me.I have only one 120 ohm resistor which was filled into master device. And I have also R2 and R4 for every device which are 47K ohms. I did not understand your calculations because, I did not use mutual GND. if you are right, acording to your calculations, I have a 4,7k ohm with 10 devices. But, it doesn't look a problem?Actualy, as I was doing my experiments, I had no oscilloscope by my side but, in the next future, i can look for A and B line with scope.TI documentation says that there must be two therminal resistor? but I used one? (I did not read it clearly, just looked at important points from everypage. I will read it clearly)

author: rstofer
author_rank: Super Contributor
timestamp: September 28, 2016, 07:34:39 pm
content: Quote from: Karamel on September 28, 2016, 03:46:46 pmYour schematic shows R2 and R4 as 560 Ohms. Neither of these resistors are required - anywhere. See the document for reasons you might want to use just one set. The drivers provide active pull-up and active pull-down.The RS485 scheme is meant for a bus of several thousand feet. The terminating resistors are to be installed at each end of the bus and never in between. In many cases, the first terminating resistor is at the RS232->RS485 adapter and the second is at the far end of the bus, even external to and beyond the last device.There is also a discussion of ground loops in that document. The section might be worth a read. You want to try to avoid ground loops and connecting power supplies together through excess pull-up and pull-down resistors might not be helpful.When I built my small RS485 bus, I had two DIN 5 jacks on each device (one for cable coming in and one for cable going out) so I could just cable the boxes together with DIN 5 connectors. The final terminator was built into a DIN 5 plug used on the last device. There were no terminating resistors inside the devices themselves. If I added or deleted a box, I just moved the terminating DIN plug.