[time-nuts] RS-232 interfacing

[M. Simon]

I'm always glad for inputs. I like learning things. 

The design decisions were made based on parts in hand for the level translators. I don't like stocking a lot of different parts. I use those for other circuits where 2 bit translation is appropriate. 

I usually pick chips based  on cost and technical adequacy. The HIN211EIAZ was near the low end. The version I use is guaranteed for 230Kbaud. Good enough for my intended use. (a microprocessor system I'm working on). Maximum baud rate needed is 115,200. My systems use a 3.3V and 5V supply. So I was not constrained to use 3.3 V only. 

Charge pump noise - you minimize that by using much larger capacitors than the minimum suggested. I do. 20X larger. 

And yes - send me some chip numbers privately. It may convince me to do a new design. If it doesn't add too much to the cost. If I can trade that off against board area so much the better. And if it doesn't complicate routing too much. I like to keep vias to a minimum to keep the ground plane as intact as possible. Minimum chip pitch I work with is .65 mm between pins. 

And then there is the question of board lay out. The chips I chose made that easy. 

As per usual - everybody has their own ideas on "best". It all depends on the elements you consider. Mine was not maximum available performance. It was adequacy for the purpose. With some margin. You know the deal. Glass half empty - glass half full - glass too big.  

And thanks for e-mailing me directly and ccing  the list. I subscribe to the digest - my time is limited. Although I may misunderstand how this list works. Wouldn't be the first time. 


 Simon

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



>________________________________
> From: Herbert Poetzl <herbert at 13thfloor.at>
>To: M. Simon <msimon6808 at yahoo.com>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>Sent: Monday, August 5, 2013 2:06 PM
>Subject: Re: [time-nuts] RS-232 interfacing
> 
>
>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)
>
>best,
>Herbert
>
>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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>