SN75176 just works OK the first time...

Carlos de Leon

Other Parts Discussed in Thread: MAX232

Dear,

I have a daisy chain network (RS-485) that it will have in the future 20 ULs and 100 meters long.

I did a test with 4 ULs using MAX481; the network has both end resistors of 120ohms and two bias resistors of 560ohms. If I send some bytes from the PC to the microcontroller the communication is OK. As I am not able to buy MAX481 transceivers in Argentina because it is not available, I decided to use the TI interface, SN75176BP.

The problem is that I did the same test with the SN75176BP and the first time that the PC send some bytes to the UL, it response OK but the second time and the rest, the UL starts to fail not receiving corrects bytes. I tried without the bias resistors because with them not even the first time receive the bytes.

Could the impedance of the line be the problem? If it is the problem why just the first time it works fine, and why if I program the microcontroller to send byte to the PC, it receive them OK (using the SN75176BP).

From the PC I have an interface made it with a MAX232 and a SN75176BP, I check the MAX232 and it is working properly. I have checked connecting the PC to the microcontroller through the MAX and everything works goods.

If you have any suggestions I really appreciate it.

Yours sincerely,

over 11 years ago

0 Bart Stiller over 11 years ago

TI__Expert 5400 points

Carlos,

Keep in mind that the biasing resistors add to the bus loading seen by the driver. With twenty 1UL receivers in addition to 560ohm bias resistors, the differential output voltage will be decreased.

Otherwise, I don't immediately see a problem with the test configuration you have described. Could you confirm the cable used in your test setup (100m? 120ohm twisted pair?). Additionally it would be useful to see oscilloscope captures at the driver output and receiver input during a failed transmission, if you could provide that information --

0 Ron Michallick over 11 years ago

TI__Guru**** 164976 points

Carlos,

Can you provide waveforms of the RS485 bus signals?
I would not expect a problem, so seeing the waveform should be helpful.

0 Carlos de Leon over 11 years ago in reply to Bart Stiller

Prodigy 45 points

Bart,

This is the datasheet of the cable: http://www.sycelectronica.com.ar/cables/CMXXC.pdf (4 vias) and it has 100 meters, I do not have more information.

Now I trying to get an oscilloscope and once I have the captures I will send them to you.

Best regards,

0 Carlos de Leon over 11 years ago in reply to Ron Michallick

Prodigy 45 points

Ronalds,

Thanks for your reply, once I have them I will attached to be analyzed.

Thanks you very much.

Best regards,

0 Carlos de Leon over 11 years ago in reply to Ron Michallick

Prodigy 45 points

Dear Bart and Ronalds,

After I had analysed the signals with the oscilloscope I found out that the problem was not unrelated with the impedance of the channel. My firmware needs to be more robust.

Thanks you very much for your help.

Interface

Interface forum

SN75176 just works OK the first time...