converting rs-232 voltage to current in my circuit design

[agent38]

Im using a TI Transimpedance Amplifier OPA138 in a design. The amp is used to convert input current to voltage output.

For reasons related to quick testing capabilities I want to take rs232 voltage signals (from my laptop) and input these signals into the amp. But the amp like I said before is looking for current (10mA max).

So my question is since the serial signal is voltage based can i just place a series resistor between the rs232 signal source and my amp to provide the current that the am is looking for?

I calculated 5k resistor foe 2ma current input. I measured 10V serial voltage.

[JoshMac]

If i understand your problem, no, no you cannot.

Are you looking to have the output of the amp analog (variable voltage), or a digital output (on or off)

If you want a analog output, you are better off using the audio output on your sound card and outputting a frequency modulated sound wave and then running to a freq to current or voltage circuit.
if you just want a single output, on or off, then I would recommend using the CTS pin on the com port, running through a NPN transistor, which you can set to deliver the current you desire.

RS232 logic is reversed and also a higher voltate than TTL logic.
There are different types of RS232 to USB adapters. The cheap ones (and RS232 on most laptops) are generally -5v (1) to +5v (0) which is why some circuits dont like them, because the 5v isnt enought to trigger a change in logic status.
The more expensive ones and desktop PC's (and MAX232 IC's) are -15v (1) to + 15v (0)

TTL
0 = 0v
1 = 5v
RS232
0 = 7 to 15v
1 = -7 to -15v

I will mention to be very careful with RS232 ports on laptops, if you sink current too high you will damage the port.

Hope this helps,
Josh