RS232 logic voltage levels

[Vicky123]

Hi All,

I want to interface an SoC to a Sensor Board through RS232.
The Soc has RS232 which is 3.3v( not in data sheet, Measured on my own).
The sensor board has standard 5v RS232( measured voltage is -8.5v , I think its correct as it can have +12/-12 right?)

Can I connect these two interfaces? If not, why? and what can be the solution?

Thank you.

 

[macboy]

RS-232 is a physical transfer medium (think OSI layer 0 if you are familiar with that) for UART commumications. The UART would be like layer 1.

RS-232 specifies voltages of -3 to -15 volts for a '1' bit, and +3 to +15 volts for a '0' bit (up to -25 or +25 volts under EIA-232 standard). Note that the polarity is inverted.

The UART device itself usually operates at TTL levels, with a '0' bit being around 0 volts (0 to 0.8 V) and a '1' bit being around 5 volts (over 2.0 V). Generally a driver chip or chips like a MAX232, or the MC1489/1488 pair are used to translate the TTL levels into the high voltage, inverted, RS-232 levels.

A UART idles at "1", so at TTL levels, that is "hi" voltage (over 2V), and at RS-232 levels, it is the "low" voltage (-3 V or greater).

If you have measured -8.5V at the sensor board, it is using RS-232 levels. Your SoC seems to be using TTL levels. The easy way to interface them is a MAX232 or similar chip (there are many clones and variations from many manufacturers). You could also build a level-shifting inverter with transistors, but you'll need +/- 12 V (or so) supplies. The MAX232 generates its own HV supplies from +5V. You can buy a board with MAX232, DB9 connector, and jumper wires on ebay for a few $ (like this - no affiliation)

[Andreas]

RS-232 specifies voltages of -3 to -15 volts for a '1' bit, and +3 to +15 volts for a '0' bit

The +/-3V specification is at the receiver.
The transmitter has to deliver minimum +/-5V into a 3-7 kOhm load. (5kOhm nominal).
Witin a 3.3V supplied system I would use a MAX3232 which is specified for 3.3V operation.

With best regards

Andreas