two-transistor serial interface in data sheet - any issues?

Bill Henning

Hi,

I just wired up the two transistor circuit from the Propeller data sheet, and it does not work for me :-(

It does reset the propeller, however PropTool v1.2 reports that it could not find any propellers. I've checked my circuit against the schematic several times and cannot spot an error.

Could it be due to only 3.3Vdc available for the circuit?

I tried it with both a PC 9-pin serial port and a USB-2-SER adapter; I've verified both ports work by using a null modem cable between them and hyperterminal.

I know its not the prop board as it works fine with another prop-plug like USB adapter I wired up long time ago.

Any ideas?

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Mike Green

It's probably the voltage levels. The 3 transistor circuit only provides +3.3V and - something. The negative voltage is supplied by the DTR line's idle voltage. For a lot of serial ports, this is marginal at best. Most serial ports use +5V / -5V. I would seriously recommend using a MAX3232 instead. The 3 transistor circuit is ok when it works, but is not reliable for general usage.

Bill Henning

Thanks Mike,

Unfortunately that confirms my fears... time to get some MAX3232's.

UPDATE:

The three transistor circuit does not work for me either.

Mike Green said...
It's probably the voltage levels. The 3 transistor circuit only provides +3.3V and - something. The negative voltage is supplied by the DTR line's idle voltage. For a lot of serial ports, this is marginal at best. Most serial ports use +5V / -5V. I would seriously recommend using a MAX3232 instead. The 3 transistor circuit is ok when it works, but is not reliable for general usage.

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www.mikronauts.com - a new blog about microcontrollers

Post Edited (Bill Henning) : 4/24/2009 3:53:15 AM GMT