max3232 to OR gate driving an LED

[popoyboys]

Hi,

I would like to seek your advice on this circuit.
What are the challenges you think would arise from this circuit?
Any suggestions for improvement?

[robotai]

Do you know what MAX3232 do? I don't know MAX3232 has any pin with Rx, RTS.
It should be for level conversion between TTL voltage level from/to RS232 voltage level. No idea about what you want.

[Sideshow Bob]

Can you post schematic that uses pin numbers, which IC is the OR gate, what kind of LED do you plan to use? As it is now your circuit will probably not work

[popoyboys]

My apologies. It was a mistake on the labels for the MAX3232.
The pins from MAX3232 should be R1OUT and R2OUT.
http://pdfserv.maximintegrated.com/en/ds/MAX3222-MAX3241.pdf

The OR gate is dual 2-input OR gate.
http://www.nxp.com/documents/data_sheet/74HC_HCT2G32.pdf

The LED is an 0805 SMD.
http://www.kingbrightusa.com/images/catalog/SPEC/AP2012CGCK.pdf

[hate]

What is your intention with that design? Make the user aware of an incoming transmission from 2 RS232s? It seems like it will work out of the box but what do you plan to do exactly?

[Sideshow Bob]

I can not find the pins named R1OUT and R2OUT. No matter that the Or gate should be connected to the TTL/CMOS Receiver Outputs and TTL/CMOS Transmitter Inputs I guess. The LED need a series resistor to limit the current. To keep things on the safe side. I would have used a LED current equal to 15mA. The forward voltage drop (LED) will then be close to 2 volts. The series resistor will then need to drop Vout(or gate)-2 volt  

[pickit2]

the max2323 will drive led's with just a resistor, there are loads of examples via google.
one of which is attached.

[Parmin]

First, I think the Maxim chip is RS3222, R1Out is pin 13 and R2Out is pin 10.
Would the idea of the circuit is to have an indicator of data received?
The circuit looks workable, but damned if I know why it is needed.

[hate]

The pinout of MAX3232 is on page 12 in the datasheet and corresponding pins are 12 and 9 for R1OUT and R2OUT respectively. Although I believe the idea here is to signal during an ongoing transmission, that doesn't actually mean a valid transmission will occur. Yet there is the problem of how the actual receiver will be connected. If a TTL input OR gate is to be used, a parallel connected (with the OR gate) receiver might not function correctly because of the current the OR gate will sink at high speeds and the output of MAX3232 might need to be buffered (or the input to the OR gate).

PS: I didn't realize a series resistor was missing at the output of the OR gate. The resistor should be there exactly as 'Bob' said.

[robotai]

It seems either TTL(74HCT2G32) or CMOS(74HC2G32) now all use MOSFET internally in input side of OR gate. The sink current in input side should only takes under 1uA. So it shouldn't cause any issue when parallel it with actual circuit in normal RS232 application (communication speed less than 500Khz). Or am I wrong?

Put series resistor with LED to make output current less than 10mA should be OK, in my opinion.

[Ichan]

Two diodes and a resistor before the led instead of OR gate is enough.

-ichan

[hate]

It seems either TTL(74HCT2G32) or CMOS(74HC2G32) now all use MOSFET internally in input side of OR gate. The sink current in input side should only takes under 1uA. So it shouldn't cause any issue when parallel it with actual circuit in normal RS232 application (communication speed less than 500Khz). Or am I wrong?

Correct except that 74HCTxxx series are not TTL. HCT is short for High-speed CMOS with TTL compatible I/O meaning they have low V_IH and high I_OH satisfying TTL compatibility. What I was referring to as TTL was something like this:
http://eecs.evansville.edu/home/resources/pdf/dm7432.pdf

Two diodes and a resistor before the led instead of OR gate is enough.

Practical but might still cause trouble in case of parallel connected receiver if not buffered.