When transmitter is disabled, on LT1785, there is current source pulling the -DATA pin to VCC through a 100Ω resistor to ground. I replaced the 1785 with a ST4E1240DT and the 100Ω pulled the -DATA pin to ~300mV. Changing back to the 1785 my problem ret

Hi Terry,

Could you please provide a schematic so we can understand your connections better? In addition, can you please also provide the top mark of the IC has high leakage current.

Best,

Here ya go!

PDF

407 1785I

On the schematic we are looking at Com3, U4 JP1 , measurments are at J5.

Jp1 is 3-4,5-6

Hello Terry - I am unable to read the schematic you attached before as it was just a blank file, could you resend the area showing the LTC1785. I will order some of these ICs from our boxstock to see if I can replicate your issue on the bench.

• Schematic take two!PDF

Just looking for any updates.. Are y'all able to reproduce the issue?

I am not clear on the circuit or the problem statement based on your description above - can you use the pin names or signal names that appear on the schematic, for example with U4 do you mean A and B pins for +D and -D?. Looking at U4 and JP1 I think you have a 100ohm from A to VCC and 100ohm from B to GND. If connector 7-8 is open then you do not have a termination resistor between A and B pins?. In addition, you connect pin A of U4 to pin 18 of the MAX211 which has an internal 5k ohm to ground. Do you have a scope shot for pins A and B for the LTC1785 ?

Yes, you are correct. U4 pin 6 is "A" or "+D" and U4 pin 7 is "B" or "-D". I isolated the MAX211 by removing pins connected to U 4 pins 6 &7. U4 pin 6 is acting as I expect.

U4 pin 7 is not. I expect pin 7 to be low when "TE" is low (U4 pin3).

In the photo below, Yellow is "TE", U4 pin 3.

Blue is "A", U4 pin 6. (Pulled up to VCC thru 100Ω) looks correct

Pink is "B", U4 pin 7. (Pulled down to GND thru 100Ω) should a square wave.

And again, a ST4E1240DT works as expected.

Yes, you are correct. U4 pin 6 is "A" or "+D" and U4 pin 7 is "B" or "-D". I isolated the MAX211 by removing pins connected to U 4 pins 6 &7. U4 pin 6 is acting as I expect.

U4 pin 7 is not. I expect pin 7 to be low when "TE" is low (U4 pin3).

In the photo below, Yellow is "TE", U4 pin 3.

Blue is "A", U4 pin 6. (Pulled up to VCC thru 100Ω) looks correct

Pink is "B", U4 pin 7. (Pulled down to GND thru 100Ω) should a square wave.

And again, a ST4E1240DT works as expected.

Hi Terry - thanks for the clarification. Just to be clear, do you see the same problem on the channel with U5?. Also when you swopped U4 out and in, did you use a new LTC1875 or resolder the previous chip (in case the original chip had a fault).  I am putting a LTC1785 onto a standard half duplex eval board with 100 ohm pullup/down resistors on the AB lines to test this chip as you show in the above scope shot. Please note tomorrow is a holiday at ADI.

Terry - I tested a LTC1785 on a EVAL-RS485HDEBZ eval board, VCC = 5V, /RE = 5V, DI = 0V, and drive DE with a 1khz square wave, with 100 ohm pull up and pull down resistors to replicate your circuit. The part performed as expected with AB unloaded and with a 120ohm load across AB. Scope shot has CH1 = DI, CH2 = A and CH3 = B. I think the device you have is faulty - have you ordered a free samples from ADI and tested those ?

Yeah, that's what I expected to see. Going to replace 1785 again just to see if I had a couple of bad parts.

Ok, I found it. The PCB had an open. The fact that the open was happened on both channels, in the same way, makes me think there is a jumper combination that will cause the problem...

Thanks for your diligence in researching this issue.