RS-485 data Receiver Issue

[NK.1]

Hi,

We are using ST3485EIDT and ST3485EBDR RS485 IC in our Gateway device to collect the data from Slave devices.

We are reading 30 parameters (1 Packet) from slave Device. Frequency: 1 parameter per sec. Baud Rate 9600 bps.

We are noticing some issue while reading the data from the slave devices. Mean, some Parameters are some times not receiving, but most of the time we missing parameters, we can't guess which parameter to be missed. In missed parameter we are printing the reasons. reason will be Extra data size, Slave not responding.

Even same issue happened, even if we read only 1 Parameters. Error rate will be 20 to 50%. Mean out 30 parameters we missed 10 to 20 parameters. If we are reading only one Parameter, out of 100 readings 50 reading are errors only, either data length Float or slave not responding.

But same slave device modbus connected with PC using USB to Modbus Converter all the 30 parameters received without fail, all the time.

We had replaced the ST3485EIDT with ISL3178EIBZ (Renesas Make) in 4 boards, All 4 boards working fine with no errors/issue.

I have replaced 220 Ohm ferrite bead with 0 Ohm Resistor and also we tried with populating 523 Ohm Resistor on R12, R13 in ST3485EIDT boards. But printing same issue.

Am wondering why ST3485EIDT not working in our board even though ISL3178EIBZ working?

Please refer the link for ISL3178EIBZ datasheet

https://www.renesas.com/us/en/document/dst/isl3170e-isl3171e-isl3172e-isl3173e-isl3174e-isl3175e-isl3176e-isl3177e-isl3178e-datasheet

Please find the schematic FYI. Kindly advice to resolve this issue.

Thanks and regards,

Naveen K

[Peter BENSCH]

RS-485 defines different unit loads: the first RS485 transceivers had a unit load of 1 and could operate on buses with 32 nodes. Later, transceivers with a higher input resistance of the receiver were developed, which could then work with 64 (1/2 load), 128 (1/4 load) or 256 (1/8 load) bus participants.

The ST3485EIDT has an input resistance of 24kohms and can therefore work on buses with 64 bus users.

The ISL3178EIBZ, on the other hand, has 150kohms and can work with up to 256 participants.

A 64-user RS485 transceiver logically places four times as much load on the bus as one for 256 users. In this respect, the ST3485EIDT is not directly comparable with the ISL3178EIBZ. Instead, you can consider the STR485LVQT, which has a different housing and is not a drop-in replacement.

Hope that helps?

Regards
/Peter

[NK.1]

Hi Peter,

Thank you for the information.

As advised by you, What we understood is we can connect upto maximum 64 slave-devices to our ST3485EIDT device and we can read any number of parameters in each devices. Kindly correct me if my understanding is wrong.

But we are connected only one slave device (Energy Meter) and reading 1 to 32 Parameters. Even if we try to read only one parameter also not receiving all the time. Please advise to resolve this issue and please advice the logic behind this concept.

Kindly advice us, How to make our ST3485EIDT device works. and also advice any changes/modification required to make it work? Mean in which condition ST3485EIDT device will works.

Thanks and regards,

Naveen K

[AScha.3]

Hi,

You should terminate with "standard" values, 3 r from gnd to vcc, 

and just short the 2 x 220r in inputs (= 0 ohm) .

And D+ goes to the more negative biased line - you sure have not mixed up ?

+ the "maximum load level" has no meaning here, you just have one load.

+ the difference between the transceivers here is there switching level; 

STM +200mV/- 200mV (standard), renesas -50mV/-200mV  ---this might be the reason for different behavior.

[NK.1]

Hi Ascha and Peter,

Thank you for the information.

We have tried with the suggested circuit/values. we are getting same errors/issues i.e data size is extra (extra byte receiving).

Kindly advice us, How to make our ST3485EIDT device works. and also advice any changes/modification required to make it work? Mean in which condition ST3485EIDT device will works. In which application?

Thanks and regards,

Naveen K

[NK.1]

Hi Peter,

Could you please advice for my previous query.

Kindly advice us, How to make our ST3485EIDT device works. and also advice any changes/modification required to make it work? Mean in which condition ST3485EIDT device will works. In which application it will works and what are all the limitations?

Thanks and regards,

Naveen K

[NK.1]

Hi Peter,

Could you please advice for my previous query.

Kindly advice us, How to make our ST3485EIDT device works. and also advice any changes/modification required to make it work? Mean in which condition ST3485EIDT device will works. In which application it will works and what are all the limitations?

Thanks and regards,

Naveen K

[Peter BENSCH]

Which RS-485 transceiver is used on the remote station (slaves)?

Please show us an oscillogram of the signal curve on the differential RS-485 line, preferably with the associated logic level on the second channel.

Regards
/Peter