[time-nuts] RS-232 interfacing

Herbert Poetzl herbert at 13thfloor.at
Mon Aug 5 10:06:24 EDT 2013

On Mon, Aug 05, 2013 at 06:35:25AM -0700, M. Simon wrote:
> A while back some of the folks on the list were discussing
> RS-232 interfacing. 

> I may have something useful for those of you still wrestling
> with the problem. An RS-232 interface (Male or Female DB-9)
> that can take any voltage from 1.8 to 5V (nominal) and turn 
> it into full RS-232 levels at 230,400 baud or less. 

> It produces +/- 10V RS-232 levels (nominal - about +/- 8V
> actual).  

> You can configure the board for DTE or DCE with a jumper 
> field (a header on .1" ctrs - so you can use jumper shorts). 

> All the usual control signals are wired on the PCB except 
> for RI. 

> There are holes available for that though. 
> It requires a 5V supply.

> The design (schematics and parts list) is available at 
> http://spacetimepro.blogspot.com/2013/07/ttl-to-rs-232-db9-m.html 
> and you can order bare boards if that would be of use to you. 

> I also have links there to the RS-232 wiki. 
> And a few other useful things.  

Please don't take this the wrong way (I'm sure you've put
some efford into the design and I'm glad that you provide
the schematic and layout as well) but I have to add a few
constructive comments on the choice of components as well
as the entire design:

 - The HIN211 from Intersil
   * low data rate 120kbit/s max
   * Capacitor Charge Pump (noise)
   * requires 5V input for 10V swing

There are plenty of charge pump based, fully rs-232 compliant
interface chips (intersil, maxim, ti, analog devices) which
work with 3.3V and manage up to 1Mbaud, and if you want to
avoid the charge pump (for noise reasons) you can pick the
version with +/-12V inputs.

 - The SN74LVC2T45CTR or similar
   * 20ns/V rise and fall
   * 4 chips are required

There are a number of level translators designed for uni
and bidirectional translation between 0.8-5V and 3.3V
which handle 7/8 or more lines in a single device.
(note: no bidirectional translation is required for rs-232)


PS: if you need part numbers, just let me know, I'm sure 
    I can dig out a bunch from my previous projects.

> Simon

> Engineering is the art of making what you want from what 
> you can get at a profit.
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