help with TTL / RS232 level converter (MAX211)

Go To Last Post
6 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hey, I'm trying to connect my AVR to a computer serial port using TI's MAX211 chip. I can send data from the computer to the MAX211 to AVR, but I can't receive data from the microcontroller. :(

For the MAX211 chip, should the V+ pin be +10V, and the V- pin be -10V? (With Vcc = +5V)
My multimeter readings:
v- = .026v
v+ = 3.82v
vcc = 4.907v

ascii schematic
AVR TX0 ==> DIN1 MAX211 DOUT1 ==> DB9 pin 2
AVR RX0 <== ROUT1 MAX211 RIN1 <== DB9 pin 3

Max211 datasheet:
http://focus.ti.com/lit/ds/symlink/max211.pdf

Thanks for your help :D

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Your V- and V+ readings seem very low.

I've not used the MAX211 before but have used plenty of MAX232's which are a similar chip, typically I've found the V- and V+ voltages to be in the region of (+ or -) 7 volts.

You have the chip wired as shown on page 10 of the app note I assume ? The capacitors are important as they form part of the charge pump to generate the voltages needed for RS232.

The other thing to watch is that if you are using the V- for anything else in your circuit, such as a negative bias voltage for LCD display contrast adjustment, you need to be careful not to draw too much current as this will cause the voltage to drop.

Could also be duff MAX211 I guess - I've replaced many a MAX232 in my time ! (not a criticism of Maxim's products btw, simply due to the fact that on any circuit it's normally the physical link to the outside world that gets a bashing due to static discharge etc...)

HTH
Robin

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

mooseydoom wrote:

For the MAX211 chip, should the V+ pin be +10V, and the V- pin be -10V? (With Vcc = +5V)
My multimeter readings:
v- = .026v
v+ = 3.82v
vcc = 4.907v

chip is powered down??

SHDN and EN must be low.

Peter

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Yeah, I have it in a circuit as on p10, with SNDN and EN connected to ground. I found the C2+ pin (15) was accidently shorted to ground through a solder bridge, but even after I fixed that, there's still no serial output. The voltage readings still seem low/off. Could my chip be dead?

vcc 4.89
v+ 3.851
v- -4.375

c1- 4.82
c1+ 4.552
c2+ 2.28
c2- -2.16

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Those V+ and V- readings seem a bit low. Are you using an RS232 to USB converter cable? I've had two brands that drag down the RS232 like that. If so, take your readings without anything plugged into your micro-P.

Go electric!
Happy electric car owner / builder

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Yep - new chip.... I reckon that the 'magic smoke' has departed :cry: