Permissible Voltage Levels to UARTs on SB70LC Board?

[lightning1]

My question pertains to the Netburner SB70LC Serial-to-Ethernet Server board.

In the data sheet, Note 1 states that "All input/output lines are 3.3V maximum". However Note 2 says "All UART signals are TTL level, external level shifters may be added for RS-232 or RS422/485 operation".

Question: Can I safely apply +5V TTL level signals to the UART0 or UART1 Receive lines, which are pins 11 and 13. Or do I have to use some circuitry to interface the 5V signals with the UART input lines?

Thanks.

Dave G

[v8dave]

Hi Dave,

If I where you, I would just use an RS232 driver that is rated for 3.3V such as the MAX3232 and this will guarantee that it will work and save you having 2 supplies. You can then run it from the same supply as the SB70LC board.

Regards
Dave...

[rnixon]

Consider the input power voltage of the device: 3.3V. So if you apply 5V on any pin of a 3.3V device, it needs to shunt the power somewhere, which is not optimal, although some processors can do it. The 5270 is not 5V tolerant, so if you apply a voltage above 3.3V to it on any pin you will damage the processor.

TTL voltage levels are: 0V to 0.8V = low, 2V to Vcc = high. Vcc doesn't have to be defined as 5V.

[lightning1]

Thank you for the prompt replies.

I had not checked the specs on the 5270 to see if it was tolerant.
Thanks for the clarification.

I'll also look into the MAX3232.

Dave

[Ridgeglider]

If you are just prototyping, or are not spinning your own boards these are small, very convenient $12 RS232 level shifters boards that work on 3.3v-5v input power: http://www.acroname.com/robotics/parts/ ... -CONN.html. I think they are based on the Intersil ICL3232, similar to the maxim parts.

[pbreed]

The SB70LC is NOT 5V tolerant.
3.3V levels only. I need to hunt down and kill the TTL comment, where was that?

Paul

[lightning1]

Paul,

Thank you for your comments. I found the reference to TTL very misleading.
The TTL acronym has been around for over forty years and has always implied a Vcc of 4.5 to 5.5V.

However, their usage of the TTL reference is one-sided. The SB70LC UARTs will output TTL "level" signals, but they cannot input TTL level signals. i.e. nothing above 3.3V.
In my mind, any reference to TTL implies that levels up to 5.5V can be tolerated.

Anyway... to answer your question, the reference to TTL shows up on the SB70LC data sheet in two places:
1. On page 1, under Features, it says "2 serial ports with TTL support".
2. On page 3, in the section "SB70LC Pinout and Signal Description", in note 2.

Dave

[pbreed]

We are changing the docs to read LVTTL 3.3V max.

Paul