RS232 Voltage Levels

[snip]

Space's brother ?:-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

I've got a datasheet for a chip that takes an RS232 serial input with a protocol of 9600,N,8,1. It indicates the voltage level at this input is hardware-selectable to either +/- 12V(as found from a PC serial port), and either normal or inverted TTL level(0-5V). I plan on feeding this chip a TTL level serial stream by bit-banging it rather than using a UART. I am familiar with the polarities and V requirements of the "real" RS232 as shown in the example below for ASCII 65("A"):

+15V-|-------------------- | ___ _______________ ___ | |S | | | | | | |T | | | | | | |A | | | | | +3V-|--|R-|--|--------------|--|--|-------------- | |T | | | | |INDETERMINATE | | | | | | |VOLTAGE 0V-|--|B-|--|--------------|--|--|-------------- | |I | | | | |REGION | |T | | | | | -3V-|--|--|--|--------------|--|--|-------------- | | | | | | | | | | | | | | TWO | | | | | | | STOP | | | | | | | BITS ____|__| |__| | | | | |__| |__|__| | | | | 0 1 0 0 0 0 0 1 0 1 1

-15V-|------LSB------------------MSB--------------

Bit width = 1/9600 seconds = 104.2 microseconds

Can someone please educate me as to what are:

1)"normal" TTL RS232 voltage levels 2)"inverted" TTL RS232 voltage levels

TIA Charlie

There is no such thing as "TTL RS232 voltage" levels.

UARTs send/receive a logic "1" (e.g. 5V or 3.3V) for a mark, and a logic "0" (e.g. 0V) for a space.

You really don't know what it means to invert a TTL signal?

--
Grant Edwards                   grante             Yow!  Yow!
                                  at               
                               visi.com

___

Thanks Mark, for clearing that up. Does this look like the proper way to send the chip an "A", then, with the mode set at "normal" TTL(0-5V)? What are the indeterminate voltage limits? Any need for a delay prior to sending the next character"?

|-------------------- +5V_|___ ___ ___ ______ | |S | | | | | | | | | | | | |T | | | | |2 STOP | |A | | | | |BITS +?V-|--|R-|--|--------------|--|--|-------------- | |T | | | | | | | | | | | |INDETERMINATE | |B | | | | |VOLTAGE | |I | | | | |REGION | |T | | | | | +?V-|--|--|--|--------------|--|--|-------------- | | | | | | | | | | | | | | | | | | | | | | | | | | | | 0V_| |__| |--|--|--|--|--| |__| | | 0 1 0 0 0 0 0 1 0 1 1 LSB MSB

Bit width = 1/9600 seconds = 104.2 microseconds

"Sic hoc legere scis nimium eruditionis habes." (If you can read this, you're overeducated.)

Who's Mark?

You'll have to look at the electrical specs for the UART in question. For standard TTL, a "0" has to be below 0.7V and a "1" has to be above something like 2V. For CMOS, it's usually

1/3 and 2/3 of the supply voltage.

Not usually, no.

It looks like a 0x41 to me.

--
Grant Edwards                   grante             Yow!  .. I
                                  at               feel... JUGULAR...
                               visi.com

Must be a spurious random message being generated by those TTL RS232 signals.

Hi Charlie, have a look at the MAX232 or MAX233 (no caps).

The only thing that springs to mind from reading your post is, "What on earth does this mysterious chip do, what is it called, who makes it?"

+2.0V-|--|R-|--|--------------|--|--|-------------- +0.8V-|--|--|--|--------------|--|--|--------------

No delay should be required.

-- Dave Tweed

.... snip ...

The receiver will normally sample the incoming line at 1/2 bit time from the nominal transition. Once the stop bit has been sampled, it should immediately go into a 'hunt' mode for the start bit of the next character. This automatically yields 1/2 bit of timing slop. So there is never any point to using more than two stop bits at the transmitter.

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[snip]

RS-232 -> TTL = MC1489

TTL -> RS-232 = MC1488

These chips are still available more than 40 years after I designed them ;-)

These chips adhere to the original RS-232 spec.

Later versions, such as some of those from Maxim, are "one-sided",

5V-only devices.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Do you have a "top-10" list of Thompson's Greatest Hits?

I see your air freshener puffing away frequently on TV. How did they select thost (odd) intervals?

As shown above

mark (binary 1) 1s -12V(ish) stop bits, 1 in the data and idle time space (binary 0) is +12V(ish) start bits, 0 the data and the break signal

for +12V read "0V" logic low for -12V read "+5V" logic high

Chips like max232 and LM1488+LM1489 convert between the two. not that you'll need to do that, but the data sheets may prove informative.

Bye. Jasen

MC1488/89 RS-232... still in manufacture

MC1530/31 OpAmp... still in manufacture

MC1650/51 HS Comparator... still in manufacture

All the PLL stuff, VCO's, VCM's, phase detectors... most still in manufacture

Automotive stuff in the '60's, early '70's

USB driver/receiver for Intel

Roughly 160 custom chips (ASIC's)... the largest source of my income ;-)

I designed the combination flyback power supply / driver for the piezo pump, and the low power clock.

Two associates of mine in Columbus, Ohio, did the "microprocessor". The odd intervals probably came from how rudimentary the uP is.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

There is no such thing as a TTL RS232 level; there are the nominal TTL levels (0 to 0.8V for a low or "zero" and 2.4V to 5.0V for a high or a "one"), and there are the levels for RS-232 which you have so elegantly demonstrated. There may still be some devices that will convert RS-232 levels to TTL and some that go the other way.

I believe a logic 1 is negative and a logic zero is positive as per the elegant diagram provided by the O.P. . For a teletype, i think (it has been a few daze) a mark was current flowing for a logic one.

EIA RS232 spec says that the max. voltage level is 15V. It also says that the minimum non-space voltage level is (7.5V?), but most devices will work at lower levels so TTL signal levels work but not be in conformance with the standard.

On Fri, 06 Jan 2006 12:21:56 +0200, Paul Keinanen Gave us:

Can you say "False low"?

360mW typical ;-)

Keep in mind this was early '60's.

Quite a few process capability improvements since then.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

How long distances do you intend to transfer the TTL signal ?

Is there a cable between the devices or is this just a PCB track above a continuous ground plane ?

The standard TTL maximum receiver "0" state voltage is 0.8 V, so if you have some ground potential differences, such as ground bounce due to fast peak currents or feeding both equipment from a single DC power supply with common 0 V causing large ground return currents, your "0" state noise marginal can be quite quickly ruined.

If you are using bit banging, you usually have some degree of uncertainty about your timing. I would suggest that you on transmit generate at least 2 (or even more) stop bits and on receive, always accept a new start bit after 1 stop bit (or even 0.5-0.7) stop bit time, regardless what the specification says about the number of stop bits in each case. This ensures a reliable connection even if the sampling points are a bit off and only reduces the throughput about 10 % on transmit.

Paul

No. TTL didn't use any negative voltages. Logic 0 is 2.X.

Inverting a TTL signal means you convert a Logic 0 to a Logic 1 and vice versa. A Boolean "not".

He's asking about TTL. I think.

--
Grant Edwards                   grante             Yow!  It\'s OKAY --- I\'m an
                                  at               INTELLECTUAL, too.
                               visi.com