I am trying to create a UART to isolated RS485 port to communicate over Modbus, both Tx and Rx LEDs are glowing together and I am getting checksum errors. The schematic is attached for reference. What could be the reason for the issue.
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\$\begingroup\$ We are getting lots of questions about similar really strange RS485 schematics lately - this is quite similar. Where are all these coming from? Is there some bad online source everyone decided to copy? For example why are there pull resistors and fuses on the differential lines and how does any of that make sense? \$\endgroup\$Lundin– Lundin2025-02-17 09:04:04 +00:00Commented Feb 17 at 9:04
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That's not even an RS-485 transmitter.
You are using it as a pseudo-CAN kind of interface, where you either send a 0 for dominant state or don't transmit anything for recessive state.
So if you think you made an isolated RS-485 transceiver, you did not make an RS-485 transceiver and the isolated transceiver is against RS-485 specs as there is the classic mistake of isolating the transceiver but leaving it floating without a proper common ground reference from the bus that other transceivers share.
So it's not compatible with ModBus. Also the bus bias resistors are not ModBus compliant.
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U8which is a TPT7721F-SOAR act as an isolator? As far as I can tell from schematic the supplies are isolated. \$\endgroup\$Chester Gillon– Chester Gillon2025-02-16 19:05:22 +00:00Commented Feb 16 at 19:05 -
\$\begingroup\$ @ChesterGillon Thanks, for some reason I did not see that chip as it was zoomed outside the screen. So of course it is isolated. However, we don't know from where the isolated 3V3B supply comes from, and the classic mistake of not providing the bus ground reference to an isolated transceiver is made. \$\endgroup\$Justme– Justme2025-02-16 20:36:55 +00:00Commented Feb 16 at 20:36
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\$\begingroup\$ Thanks for replying, I have added the power supply schematic as well, if that clarifies \$\endgroup\$Praveen– Praveen2025-02-17 05:07:53 +00:00Commented Feb 17 at 5:07
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\$\begingroup\$ @Praveen The isolation or supplies are not an issue. However it is not shown from where 5V and 5VB come from. \$\endgroup\$Justme– Justme2025-02-17 05:09:50 +00:00Commented Feb 17 at 5:09
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\$\begingroup\$ @Justme - 5V pin of a raspberry pi is connected to the VIN of BL0505LS-1W which has the VOUT of 5VB. Raspberry pi in-turn is powered by a USB Type-C 5V 2A power supply. Does this explains ? \$\endgroup\$Praveen– Praveen2025-02-17 06:54:04 +00:00Commented Feb 17 at 6:54