my schematic is above ,on board , pin D continually send data ,now i measure the differential output of SN65HVD3082 has a ramp ,please help me
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Thank you very much for your reply! I have tried to increase R154 to 10k and add a 220pf capacitance between the gate of FET and the gnd ,but it seems to have no effect . finally ,I reduce the values of R152 and R153 to 1k ,then the ramp got disappeared. So i have a question : why is the fist method not effective ? Is the value of R154 and the 220pf capacitance not right ,or something else? how to choose the value of them? the scope above is when the R152 and R153 are all 1k.
still this circuit,ingore the previous question,now R152 and R153 is 2k,and the R158 and R159 IS MOV10D681K , the rate of RS485 is 38400bps,then it can work well ,when I connect rs485 of 27 my devices to simense 485 device(ddc),where the distance between my device and simense 485 is 30cm,and the the distance between my rs485 devices is 30 cm, the simense device can recognize all the 27 mydevice ,but when i extend the distance between my device and simense 485to 400 m impedance of which is 26.3Ω ,then the simense device can only recognize 25 devices through soft on pc . the rate of RS485 is 38400bps,all of above ,there is no 120 temination.what's the reason?could you tell me a method to solve it ?can you reply to me quickly,I find you answer my question every two day ,hope you can answer me timely,thank you!
It sounds like having so many resistances in parallel may be causing the low level signals (when the driver is enabled) to be reduced in amplitude, causing bit errors when lots of nodes are combined with long cables. Increasing the pull-up and pull-down resistances is a good solution. This would make the transition times slower if fewer nodes are used, though. If that is a problem, you may want to revise the approach so that the resistances are placed at one or two designated nodes or on the bus itself rather than at each node.
Max