Issue with voltage levels for RS-485 interfacing

I am working with MAX487 low power RS-485 transceivers and have successfully implemented RS-485 link between two PCs using RTS under software control, and level translators for RS-232.

I have a data terminal equipment (digital energy meter) with RS-485 interface, for which I am interested in reading the data. I don't know the voltage levels (differential voltage) of the meter, and the MAX487 differential voltage is 4 to 5 V. What should I do about that...? Any help will be appreciable!

Re: RS-485 interfacing

Rox said:

I am working with MAX487 low power RS-485 transceivers and have successfully implemented RS-485 link between two PCs using RTS under software control, and level translators for RS-232.

Click to expand...

Could you do a half duplex RS485 without using RTS, just with TX, RX and GND ?

Rox said:

I have a data terminal equipment (digital energy meter) with RS-485 interface, for which I am interested in reading the data. I don't know the voltage levels (differential voltage) of the meter, and the MAX487 differential voltage is 4 to 5 V. What should I do about that...? Any help will be appreciable!

Click to expand...

The open line differential voltage of your DVM will not tell you if it's or not compatible with your RS485 interface. Just because the RS485 is a differential current line interface and adapting the end of the line with resistive terminators could modify your differential voltage. Take a look on to the RS485 specification about the voltage margins.

greetings,

Re: RS-485 interfacing

melc

Could you do a half duplex RS485 without using RTS, just with TX, RX and GND ?

Click to expand...

Is it possible... How?

Re: RS-485 interfacing

what is your DVM ? do you have link to to manufacture site?

this is need to be write in the user manual of the device

RS-485 interfacing

To suppress RTS usage you have to put circuitry to switch rx->tx direction at the detection of start bit on serial . Simple 555 can be used but for every speed adjustment has to be done manually (usually via jumpers) or you have to design serial speed detection circuit to leave it unadjusted .

Re: RS-485 interfacing

artem

Your idea is appealing but I am not much clear about the serial transmission. Is this the kind of scenario that first one device will transmit, and then the other transmit while the first one is in receive mode and all this will be sequential. What about if both transmits or there are more than 2 devices on the bus?

Anyhow right now my problem is the voltage levels. The link for the meter is
h**p://www.iskraemeco.si/emecoweb/eng/product/meters/mt421.htm
The technical data does not provide details about RS-485 voltage levels!

How should I start experimenting... connecting the working MAX487 circuit of RS-485 (0 to 5 V differential) with one end connected to meter RS-485 interface and other end to the PC with RS-232 level translator? Is it possible that few initial $ or < characters will enable the meter to transmit?

Re: RS-485 interfacing

here is a good schematic:

http://www.rmv.com/documents/Converter_232-485.pdf#search="rs232 rs485 convert"


you can use transformer device from usb to rs485 or from rs232 to rs485

http://www.rs485.com/pconverters.html


or you can use multiprotocol chips and build your board:

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here is an commpration of the formats:

http://www.rs485.com/rs485spec.html


don't forget to press the "Helped me" button

thankx