Rs-232-c

[YAN-1]

Hello everyone. I was reading the datasheet of an electronic compass that I intend to purchase and I was trying to make sure whether I can interface to it through the UART module of my 16F877A. In the datasheet of the compass, it says that it has 2 forms of obtainig UART communication. The first one has RC and TX pins with voltage levels of 0V for logic 0 and 3.3V for logic 1. For the other pair of pins, it says 'Fully Compliant with RS-232-C Standard'. Which one of these should I connect to the RC and TX pins of my 16F877A PIC? Will I be needing a MAX232 converter, and doesn't the PIC module follow the normal 0V/5V TTL?

Thanks a lot. Your help is much appreciated.

Nichola V. Abdo

 

[ericgibbs]

hi yan
>> For the other pair of pins, it says 'Fully Compliant with RS-232-C Standard'. Which one of these should I connect to the RC and TX pins of my 16F877A PIC? Will I be needing a MAX232 converter, and doesn't the PIC module follow the normal 0V/5V TTL?

You will require a MAX232 connected to 16F877 PIC's TX/RX pins.

The PIC /TX and /RX are TTL levels which are compatible with the MAX232,
which will translate the signals to the RS232 standard, suitable for connection to a PC's serial port.

 

[YAN-1]

Yeah but can't I connect the PIC directly to the 0/3.3V option of the compass?

 

[ericgibbs]

>> The first one has RC and TX pins with voltage levels of 0V for logic 0 and 3.3V for logic 1.

From memory the 16F877 Vdd is specified as +5V, if will work at Vdd=3.3v I have never tried. Check the data sheet.

Also the 'sense' of the /TX and /RX maybe not be the same as the compass, does it say, can you post the compass datasheet?

 

[eng1]

Personally I would consider using pins with TTL buffers on PORTB - for example - and software UART. The treshold voltages are 0.8 V (low level) and 2 V (high level) with Vdd = 5 V.

 

[YAN-1]

There you go. Please check it out and let me know what you think.

 

[ericgibbs]

hi,
As the datasheet says the logic levels for the SPI interface are fixed at 3.3V
as are the internal UART levels, however the RS232 lines are standard.

So with a MAX232 on your PIC /TX and /RX inputs is a simple connection.

If you can find a PIC that will operate at Vdd = 3.3V and has a SPI interface you could chose that method.

I would recommend a standard RS232 interface with a MAX232.

To help to write and debug your NEMA protocols you could always connect your compass via the RS232 to a PC.

Is this OK?

 

[Sceadwian]

3.3 volt output logic will interface with 5 volt input logic nicely. As eng1 said the threshold voltage is around 2 volts. But 5 volt to 3.3 volts logic isn't as easy, some 3.3 volt chips are 5 volt tollerant, some are not, a simple voltage divider can make the down conversion.

 

[YAN-1]

I see. Thanks a lot. I think I will use the MAX and connect the PIC directly to the RS232 pins of the compass. And when I need to connect the compass directly to the PC for resting, I don't need a MAX, right? And on another matter, how does the MAX generate the 12 or 15 volts for the RS232 standard from the 5V supply? Thanks.

 

[Ayne]

One thing more..
MAX232 is also invert it's out put.
If it's input is low it will give u High output..
Keep this in mind.

 

[Pommie]

The datasheet states that the UART:URXD0 and UART:UTXD0 are non inverted 3.3V logic levels and should interface with a pic directly without a MAX232. You would of course still need a voltage divider on TXD if you are operating on 5V, however, the supply can be upto 20V.

Mike.

 

[ericgibbs]

hi yan,
The MAX232 has internal switchers, which together with 4 external capacitors will generate the +/- Volts required for the RS232 connection, the MAX voltages are about +/-9Volts.

What will be the distance between the Compass and PIC pcb??
With a RS232 link you could have a distance of around 30mtr, using a screened cable.

I dont know the application for your Compass, can you tell us???

I designed and manufactured a electronic compass for marine survey applications, it is called 'POLARIS'.
The siting of the Compass unit is important, thats one reason I am suggesting the RS232 method, also you be able to interface the Compass unit directly to a PC's serial port for debugging and test.

 

[YAN-1]

So sorry for the late reply. Well I intend to use the compass to determine the orientation of a car in an unmanned ground vehicle i'm working on. Basically, it'll be mounted on a moving car so I'm worried about the vibrations. I'm not sure yet about the distance from the PIC pcb, but it shouldn't be that far. Any advice? Thanks a lot.