What does "it seems" mean?Seem TVS is shorted to GND
Hi KLausST ,Hi
What does "it seems" mean?
Is it shorted? If yes, which diode?
How many node do you have? All with identical circuit?
Is the "protection circuit" your idea, or from an application note?
What is R5, R9 for?
Klaus
Why not measure it? Then you can be sure.I just guess it short
I don´t know what you mean with "I knew"....are referenced from some ciruit that i knew
It is diode SMAJ6.8CA not SMCJ65A ( I have not updated on circuite)Hi,
please send a link to the SMCJ65A datasheet. Best directly at the manufacturer´s internet site.
Klaus
--- Updated ---
Why not measure it? Then you can be sure.
I don´t know what you mean with "I knew"....
The circuit makes not much sense to me. It really would be a good idea to send a link to the according document.
Klaus
Please update your schematicI have not updated on circuite
That's right, if all 30 nodes are conneceted and powered. Is so, the network should work in this special case. Generally, it's required to work with 2 to 30 nodes, so you need to provide strong bias resistors at the end nodes.if I´m not mistaken, then
* the 120 Ohms resistors are only installed at both ends of the bus. So a total of 60 Ohms.
* the 2 x 20 kOhms are installed on all 30 nodes, resulting in 2 x 670 Ohms.
the bis voltage then should be above 200mV.
Hi Klaus.,Hi,
I fully agree.
so at both ends: 720 / 120 / 720 Ohms
I wonder if there is no biasing at the nodes.. while they are disconnected from the bus
Isn´t the level undetermined and prone to noise pickup?
about 150k each should at least provide a proper HIGH level.
Klaus
Sorry I don't have a osilloscope for measurement.Hi,
do you have a scope to verify the signal levels?
Klaus
Yes, but only if the protection circuit is the problem. It's still not clear.If I don't use protection circurit then is it OK ?
Hi Brian,additional
Thank for your comment. But why do i see almost use protection circuitHi,
A good transmission line has a known characteristic impedance ... and it should be linear.
Linear means that the magnitude of the current follows the magnitude of the applied voltage (twice the voltage leads to twice the current)
DC resistance of a cable is less important ...as long as the voltage levels are good at the far end (seen from the driver)
For reflections: dV/dI is more important than V/I.
Your protection circuit violates all this above.
* It causes current during normal operation (what a protection circuit should not do)
* It causes different currents for LOW than for HIGH
* It causes very unlinear current (maybe 10% higher voltage causes 100% higher current)
* it causes much higher current at the nodes close to the driver than at those far away.
* at least at nodes close to the driver the current through the protection circuit is much higher than through the termination
For sure you are free to ignore it ...but if you want reliable/stable results....
Klaus
The bias resistors are only needed once in the network, regardless of how many nodes. RS485 can have three electrical conditions: master sending to slave, slave sending to master and both ends receiving. Imagine situation where the master requests some data from a slave but the slave has to spend some time before it can reply (for example taking a measurement), after the request was sent, the master would switch to receive mode but the slave would not yet be transmitting. During that time, nothing would be driving the bus so it could go to an indeterminate state, the bias resistors are a weak force to hold the bus state in a known condition until one of the devices can drive it.
Brian.
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