mod-rs485

[crisman]

Hello, why in MOD-RS485 the resistors R1,R2,R3,R5,R6 are not populated? In the schematic they are present, but there is not the ohm value. Can you tell me which value is required? I'm trying to make work this module, but I think it cannot work correctly without these resistors. Please help me. Here is the schematic https://www.olimex.com/Products/Modules/Interface/MOD-RS485/resources/MOD-RS485-schematic.pdf

Regards

[LubOlimex]

Hey crisman,

We have provided mount pads for such resistors but we haven't mounted them since we consider them optional. The thing is that if you use MOD-RS485 only to send data it would not make any difference if you have the resistors or not; BUT if you use the MOD-RS485 only to receive data it is a good practice to have data lines set in a specific level (0-low or 1-high) - you can solder these pull-up OR pull-down resistors. If you don't do so it is possible to have unidentified signal or changing values that might confuse you while reading. Especially if you have more than one MOD-RS485, etc.

Best regards,
Lub/OLIMEX

[crisman]

Hello, thanks for your response. I would like to use this module to write and read (half duplex), so I think these resistors are needed but I did not understand from your reply if I need all of these or not and which value to choose.

Regards

[LubOlimex]

Hey crisman,

This depends on your choice and the device connected on the RS-485 line. If you connect the resistors that would connect the lines to 3.3V the line will be in high position by default (value "1"; these resistors - R1 and R6 - are called pull-up resistors). Respectively if you place resistors R5 and R2 both A and B lines would be in default low ("0") state (R5 and R2 are called pull-down resistors).

Please check the wiki article: http://en.wikipedia.org/wiki/Pull-up_resistor.

Best regards,
Lub/OLIMEX

[crisman]

I found on internet a schematic in which line A is pulled down and line B pulled up. In this way the bus is polarized to avoid interferences. Is that possible?

Thanks

[LubOlimex]

Yes, any combination is possible. It depends on your choice.

Regards,
Lub/OLIMEX