RS232 treshold revisited

Question. Is the RX line always going to be available, when the TX is required?. RS232, is never going to be a 'low power' interface. You need to be able to drive at least a 3K load, with +/-3v (at the receiver - supposedly +/-5v at the transmitter), and given that the idle condition is 'driven', you are talking a minimum of over 3mA...

Look at the DS275. Close to compliant, and the solution used in most low power devices. It steals the -ve voltage, from the incoming RX line, and uses this to drive the TX. However even with this (or your solutions), you should consider using a protocol at the PC end, which accepts 'break' between packets, and resynchronises when the packet starts, and then have your driver switch off when not actually sending. Otherwise, even with a low power solution, a CR2032, will run down too fast.

Best Wishes

There are RS422 transceivers that have an Enable signal and just take 30uA when disabled.

Rene

--=20 Ing.Buero R.Tschaggelar -

& commercial newsgroups -

The idea would be to pull down a resistor connected to a voltage "stolen' from the RS232 itself. Thus the current would not have to be supplied by the device (other than the base / gate drive).

The device will be used stand alone, and will be "docked" to the PC occasionally.

I need to have a feel whether the "TTL" method will work with e.g. 99% of the PC's out there, or a significant lower number. I never met a PC that would not accept the TTL method - but there are a lot of PC's I didn't meet :-)

I will investigate the DS275 - thanks!

--
 - René

Here's a *very* low energy solution that I came up with several years ago. I'm not sure whether it'll be cheap enough for your needs.

The first part is a 'power sucker' that draws power from the RS232 lines from the PC. It uses a switched capacitor voltage inverter chip to allow it to suck power from an RS232 line regardless of whether the line is high or low. Note that this means you can suck power from a data line which is carrying data.

It generates + and - rails to supply the RS2323 driver.

RS232 lines from PC +ve

| | | | |8

| | | | .---| | ... | | ... z --- | | Flying | | | A --- --- | cap |3 | | | | --- | voltage |---. | | | | | 4| converter| | | | === === '---| | === | | GND GND '----------' GND | | |5 | '--|

Almost all of the hand-held GPS receivers I have seen use 0-5 volt levels for the 'RS232' outputs. If Garmin et al can get away with it, you probably can too. They politely ignore complaints from laptop owners who cant get theit units to communicate.

I personally think its a really bad idea, but they do it.

--
Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
Design Engineer         J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.

There are some 3V RS-232 driver/receivers that have auto-shutdown and will draw only a microamp or so when the cable is unplugged.

I suggest using one of those rather than fiddling around.

But, and you probably don't want to hear this, RS-232 is a dying interface. Most new laptops don't have serial ports. You may wish to consider a USB interface. At least make sure your product works with one of those cheap-ass converters.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Almost all the hand-held and marine GPS receivers I've seen don't have, and don't claim to have, RS-232 outputs. They have NMEA-0183 outputs, and the NMEA-0183 spec calls for 0/+5V.

--
Peter Bennett VE7CEI 
email: peterbb4 (at) interchange.ubc.ca        
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html 
Newsgroup new user info: http://vancouver-webpages.com/nnq

Spehro Pefhany skrev:

I guess you could just turn on hardware flow control and steal the negative voltage from CTS, afaik it is negative when you are allowed to send.

yep, it maybe just as easy to just add a usb to serial converter on board instead of a max232 or similar. it also means you could get power from the pc when connected

-Lasse

In article , Spehro Pefhany wrote: [....]

Better yet, make it plug into the net work and have a little web server in it. This way any PC that can browse the web can be used. You won't have to constantly change your drivers to match the latest madness from Redmond.

--
--
kensmith@rahul.net   forging knowledge

Thanks everybody for all offered solutions so far.

The problem is that the design needs to communicate trough a docking unit over not more than 3 contacts, There is no room on both the pcb and in the budget to put in a dedicated chip. (we need to "refurbish" an existing project line in an existing housing)

So I am sort of obliged to go the transistor (fet) TTL route. Let's say the "RS232" budget is ~ $0.05 in quantities.

Given the constraints mentioned above, would one select the "negative ground" solution (device grounded to V- to give full swing), or the "true ground" solution (TTL treshold needed, but solid ground)?

Do some / all laptops have the treshold @ 0V?

again, TIA!

--
 - René

My Garmin GPSMAP76 outputs RS-232, levels are +5.5V and -5.5V.

Meindert

Ehhrrr, I think it's the other way around: they claim to have an NMEA-0883 output while in reality, they have an RS-232 output. A real NMEA-0183 output is 0/+5V AND differential (two wires going in opposite direction). My Garmin GPSMAP76 is true RS-232.

Meindert

For example ?

I have to interject that RS232 has some advantages over other interfaces especially when the datarate is low, one of which is simplicity. You can make whatever hardware with an RS232 interface talk to the device in question. A simple controller in a box or a PLC can control a synthesizer. Even a Palmtop computer can control a serial device. Whereas for USB, there is just a PC of whatever generation. The PC generations coming up a bit too quick. And you need drivers fitting to this PC and its OS. The next advantage of RS232 or its cousins the RS422 and RS485 are the cable length. USB with just 5m is a bit on the short side for industrial environments. No, the RS232 and its cousins are far from dead.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

I lied a bit...

According to my NMEA FAQ:

"The NMEA-0180 and 0182 standards say that the talker output may be RS-232, or from a TTL buffer, capable of delivering 10 mA at

4 V. A sample circuit shows an open collector TTL buffer with a 680 ohm resistor to +12 V, and a diode to prevent the output voltage from rising above +5.7 V.

NMEA-0183 accepts this, but recommends that the talker output comply with EIA-422. This is a differential system, having two signal lines, A and B. The voltages on the "A" line correspond to those on the older TTL single wire, while the "B" voltages are reversed (while "A" is at +5, "B" is at ground, and vice versa)"

My Garmin 12XL and 168Map/sounder are both 0/+5.

--
Peter Bennett VE7CEI 
email: peterbb4 (at) interchange.ubc.ca        
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html 
Newsgroup new user info: http://vancouver-webpages.com/nnq