Re: RS-232 to LVTTL

On Mon, 11 Apr 2005 12:30:15 +1200, Terry Given <my_name@xxxxxxxx>
wrote:

>John Larkin wrote:
>> On Mon, 11 Apr 2005 11:51:01 +1200, Terry Given <my_name@xxxxxxxx>
>> wrote:
>>
>>
>>>Mac wrote:
>>>
>>>>I want a quick and dirty conversion of RS-232 to LVTTL. That is, it is
>>>>TTL except that the signal cannot exceed ~3.3 V.
>>>>
>>>>Will this work (use courier or similar for ASCII art schematic):
>>>>
>>>> VCC (5V)
>>>> |
>>>> /
>>>> \ R2 (1k)
>>>> /
>>>> |
>>>> +----------------+-> to LVTTL device
>>>> | |
>>>> R1 (22k) /c /
>>>>RS-232->---/\/\/\/\--+----| Q1 (npn) \ R3 (1k)
>>>> | \e /
>>>> --- | |
>>>> D1 / \ | /
>>>> | | \ R4 (1k)
>>>> +--+--+ /
>>>> | |
>>>> GND GND
>>>>
>>>>Thanks!
>>>>
>>>>--Mac
>>>>
>>>
>>>parallel a resistor with D1, to ensure the transistor stays off over
>>>temperature with the RS232 disconnected (Icbo has to go somewhere).
>>>
>>>Cheers
>>>Terry
>>
>>
>> But transistors don't leak any more. I bet the diode leaks more than
>> the transistor does!
>
>A BC847BPN is spec'd at Icbo = 5uA at Tj=150C. 15nA typ.
>
>I recently picked up a copy of RF Shea's "Transistor Circuit
>Engineering" (its really a GE book), for the whopping price of NZ$0.50.
> The chapter on bias stability discusses using leaky diodes to
>compensate for leaky transistors over time, temperature and sock colour.
>
>>
>> But one could replace the diode with a resistor and make everybody
>> happy.
>
>yep. I'd use a single 10k quad-pack, and have a 2V5 (cf 3V3) output.

---
RS-232 can go as high as (IIRC) +/- 15V, and a B-E junction
reverse-biased at 7.5V isn't a good thing.

--
John Fields
Professional Circuit Designer
.

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