I’m developing a RS485 communication system with a master and multi slaves.
I’ve a problem with the transceiver that , when all work fine, start to be hot and after go faulty.
i used a exar SP485CN with 100ohm series resistor between pin 6 and 7 and the bus and 220 ohm parallel resistor for impedence line.
There isn’t a terminator resistor because I’m working with 1m bus, but in my test i also try with this resistor but nothing changed.
The power is 5V and i used tx_enable and rx_eenable (pin 2 and 3) connected togheter with a arduino pin.
The system work also for 10 days without error or faulty, but sometimes one or more transceiver go faulty.
I don’t understand why! I used a baud rate of 19200
Don't understand why you call a resistor between pins 6 and 7 as "series" - its a termination resistor.
There should be a termination resistor at each end of the bus, matching the impedance of the bus
(standard twisted pair is about 120 ohms or so - use 120 ohms at each end). You may want bias
However this probably isn't the problem - check the power supply voltage is stable, you do have decoupling
right at the chip's supply pins? (not optional)
Is the destination at roughly the same ground potential (there is a limited tolerance for mismatched grounds
with this sort of chip)
The chip will get warm, its supposed to dissipate about 50mW or more in normal TX operation.
the 100ohm resistors are been inserted to limite the current from tx in case of short circuit on bus, and are in series between pin 6 and pin 7 and the bus like R11 and R10 in this picture.
there are more that one 100nF to decoupling the power supply and one 100uF 16V electrolitic.
the ground potential are the same because the power supply are the same for all devices on the bus.