ISO3082: RS 485 REPEATER DESIGN NOT WORKING AT 115200 BAUD RATE.

Hi Aakash,

Sorry, I mistakenly pressed that my last post resolves your issue. Please do help me with the requested information and one of my colleagues will come back to you on this. Thanks.

Regards,
Koteshwar Rao

Observation points.xlsx

AAKASH,

Thanks for providing this data. Can you provide a schematic as well?

Respectfully,

Lucas

Dear sir

Pls find attached file

AAKASH,

Unfortunately the schematics didn't come through properly. Can you try again? If it still doesn't work then we can try emailing if possible.

Respectfully,

Lucas

http://www.ti.com/lit/df/tidrbn8/tidrbn8.pdf

AAKASH,

Thank you. I see that you are using the exact same schematic as the TIDA-00333. That reference design advertises a max data rate of 100 kbps. Does your board still have the same issue at lower speeds?

Respectfully,

Lucas

Hello all,

      After working onto this Texas circuit with the timing circuit, timing for the schmitt trigger adjusted to 8 ns the circuit is now working with ISO 3088 transreceiver in no load condition. The input waveform is reflected in the output side when no load is connected. But when i am connecting this repeater with the load unit the output waveform disappears.

 Can anyone suggest the probable cause for this happening.

AAKASH,

What is the load that you are connecting?

Hello Lucas,

    I am connecting the repeater in between the signal simulator and the slave device which is driver display unit.

Hi Aakash,

Lucas currently is out of office and I will try to support you while he is away from work.

Could you please confirm again that you are running at 115200 baud rate even with ISO3088? What bit-rate does this correspond to?
I am not quite sure why connecting the driver display unit (DDU) makes the DDU side bus waveform to disappear. Could you please share the schematic of your repeater design? The schematic you have shared earlier is a TI schematic, I would ideally like to look at your schematic even if the schematic is similar. Thanks.

Regards,
Koteshwar Rao

Hi Rao,

   Yes we are running at 115200 baud rate with ISO3088. Attached herewith is the schematic we are using for your reference and kind action.

ptrs3002_ti_du_590_rev_1_0_schematic.pdf

Hi Aakash,

Thank you for sharing your schematic, it does look exactly like the TI schematic. Hopefully there isn't any issue in the PCB layout.

I also wanted to understand the bitrate that you are running your application at? I understand the baud rate is 115200 but based on the symbol size (4, 8 or higher), the actual bitrate will be much higher. I am not quite sure how much does this design support because it was only designed for 100kbps. Please do share the maximum bitrate that you are running your application at.

One other experiments you can try out is to reduce C21 & C22 cap values. These are currently 100pF, may be you can try reducing them to 22pF or 10pF and test. This seems necessary for operating the design at higher datarates. Thanks.

Regards,
Koteshwar Rao

Dear sir

we are using bit rate of  1.5Mbps

Pls suggest in schematic to change any component according to bit rate and baud rate 115200

Hi Aakash,

Thanks for the confirmation on bitrate.

As a first setup, please do try reducing C21 and C22 to either 22pF or 10pF and test again. Let us know if this resolves the issue. If this doesn't resolve the issue then I will ask about this to a broader team and see if I can find any recommendation to try out. Thanks.

Regards,
Koteshwar Rao

Dear sir

we have tried and  we are getting wave form at output side but when we connect load at output terminal wave get disappear

Hi Rao,

   As you said we have tried reducing the values of the capacitor . But same result when connecting the load the waveform disappears.

Please suggest something that really makes the circuit operable with load connected.

Regards,

Aaaksh,

Additional loading will reduce the overall output amplitude, so it is possible that what you are experiencing is an output level that has degraded to such a point that it no longer meets the threshold requirements for their receiver.  At the system level this could appear as a loss of signal.  If that were the case the solution would likely involve modifications to the biasing and termination resistances used on the ISO308x output. Are able to provide waveforms of this disappearance?

If the signal has truly vanished entirely then I would confirm that the D and DE lines are being properly controlled.  If we could see D, DE, A, and B all together on a scope it would help.  If the DE line is high but the A/B lines do not match the state defined by D, that would be a strange issue.  I would look for things like a loss of power supply, an over-temp condition, or a short-circuit on the bus.  This investigation could reveal that the DE line is not being properly controlled, though – in that case, the resolution would more likely involve the Schmitt trigger circuit.

Respectfully,

Lucas