On Sat, 22 Dec 2012 10:28:05 +0200, upsidedown@downunder.com wrote:>On Fri, 21 Dec 2012 14:46:15 -0800 (PST), Klaus Kragelund ><klauskvik@hotmail.com> wrote: > >>> What Modbus "standard" ? >>> >>> The closest that I can think as electric Modbus standard is the >>> http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf >>> "MODBUS over serial line specification and implementation guide V1.02" >>> >>> Look at page 28 >>> >>> >Line termination may be a 150 ohms value ( 0.5 W ) resistor. > > >>Yes, the Modbus standard defines that, but the widespread industry >>standard is 120 ohms and no capacitor. (adopted from the RS485 standard) > >I still think that you are trying to solve the wrong problem. > >By using some add-hoc driver design, you are most likely going to >create much more compatibility issues with devices from various >vendors. > >Since the RS-485 standard requires a +/-200 mV swing between the >receiver terminal, with 54 ohm total load, the driver must be able to >supply _at_lest_ +/-3.7 mA. > >With +/- 2V Tx voltage swing, the total loop resistance should be >below 540 ohms, i.e 240 ohms in a single conductor, so at least 0.3 mm >wire diameter is required for 1000 m. >[snip] Which reminds me... somewhere in the basement of my mind I remember working on a bus driver which was CURRENT-drive. Upsidedown's comments suggest doing just that plus adding a voltage limiter. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Discrete custom design of RS485 driver
Started by ●December 21, 2012
Reply by ●December 22, 20122012-12-22
Reply by ●December 22, 20122012-12-22
On Sat, 22 Dec 2012 10:41:35 +0200, upsidedown@downunder.com wrote:>On Fri, 21 Dec 2012 14:41:12 -0800, John Larkin ><jlarkin@highlandtechnology.com> wrote: > >>>Do you need to tri-state the driver? If so, Larkin's suggestion >>>doesn't work. >> >>Why not? Use tri-state tiny-logic drivers. >> >> >>Even with tri-state you have to watch out for "kick" >>>above/below rails. >> >>Kick? Logic chips can't drive transmission lines? Add some protection >>if you expect lightning bolts. >> >>Logic chips have ESD diodes in both directions. Discretes usually >>don't. You're being a jerk, as usual. > >Please remember, that the RS-485 specification requires -7 .. +12 V >common mode voltage range, well above and below the usual 0, +5 V >power supply range.Whether a specific system needs that much common-mode tolerance is another issue.> >Since the driver is constantly connected to the bus, so while two >other devices are communicating with each other, the common mode >voltage can vary greatly. I hope that your ad hoc driver will not go >into some latch-up or high protection diode leaking mode, disrupting >the communication between other stations. > >I really hope that I will not encounter such substandard ad hoc >drivers on RS-485 systems that I am responsible of. >At 125 MHz, 250 Mbps, it's unlikely you will. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●December 22, 20122012-12-22
On Sat, 22 Dec 2012 08:35:48 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:>On Sat, 22 Dec 2012 10:28:05 +0200, upsidedown@downunder.com wrote: > >>On Fri, 21 Dec 2012 14:46:15 -0800 (PST), Klaus Kragelund >><klauskvik@hotmail.com> wrote: >> >>>> What Modbus "standard" ? >>>> >>>> The closest that I can think as electric Modbus standard is the >>>> http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf >>>> "MODBUS over serial line specification and implementation guide V1.02" >>>> >>>> Look at page 28 >>>> >>>> >Line termination may be a 150 ohms value ( 0.5 W ) resistor. >> >> >>>Yes, the Modbus standard defines that, but the widespread industry >>>standard is 120 ohms and no capacitor. (adopted from the RS485 standard) >> >>I still think that you are trying to solve the wrong problem. >> >>By using some add-hoc driver design, you are most likely going to >>create much more compatibility issues with devices from various >>vendors. >> >>Since the RS-485 standard requires a +/-200 mV swing between the >>receiver terminal, with 54 ohm total load, the driver must be able to >>supply _at_lest_ +/-3.7 mA. >> >>With +/- 2V Tx voltage swing, the total loop resistance should be >>below 540 ohms, i.e 240 ohms in a single conductor, so at least 0.3 mm >>wire diameter is required for 1000 m. >> >[snip] > >Which reminds me... somewhere in the basement of my mind I remember >working on a bus driver which was CURRENT-drive. > >Upsidedown's comments suggest doing just that plus adding a voltage >limiter. > > ...Jim ThompsonI usually try to analyze a point-to-point RS-422 circuit as a bipolar current loop with at least +/- 1.7 mA loop current through the 120 ohm receiver termination resistance. Compare this to the unipolar 0..20 mA (or even 0..60 mA) Teletype era unipolar current loops. RS-485 multidrop circuits are a bit more complicated. At higher data rates, the bus must be analyzed as a section of (more or less) matched transmission line.
Reply by ●December 22, 20122012-12-22
On Sat, 22 Dec 2012 07:51:27 -0800, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:>>Please remember, that the RS-485 specification requires -7 .. +12 V >>common mode voltage range, well above and below the usual 0, +5 V >>power supply range. > >Whether a specific system needs that much common-mode tolerance is another >issue. >In countries using the IEC TN-C or TN-C-S mains wiring conventions, this is a day to day issue. If there is a 4 Vrms voltage drop in the neutral conductor, the potential difference between grounded sockets ground terminal can be more than 7 Vpeak, thus causing harm to data transfer.
Reply by ●December 22, 20122012-12-22
miso wrote:> Is it possible the VC is going to stupid internet social media garbage? > Ya think? >In part, yes. Med has become less attractive, social media is all the rage. Different investors though but that's where the money flows to. Well, the money that's not kept piled in money market funds.> Why build a market for a hardware device, which hey, is work, when you > can sell nonsense. The current mania is photographs that disappear, when > it used in sexting. Hey, I know of a few congressmen who could have used > such a service. >Social media is the next bubble that will go *KABLAM* on the financial markets. I herewith predict that :-) -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●December 22, 20122012-12-22
dagmargoodboat@yahoo.com wrote:> On Dec 21, 3:48 pm, Joerg <inva...@invalid.invalid> wrote: >> Klaus Kragelund wrote: >>> On Friday, December 21, 2012 7:12:56 PM UTC+1, Joerg wrote: > >>>> Just a comment: Diodes are already in the FETs, in the form of body diodes. >>> Yes, I added parallel more sturdy diodes, to direct the current away from the low current body diodes. >> Usually they are about as sturdy as the channel in the FET, can take a >> similar current. > > Note: I don't see a spec for the FDV304P's trr. Body diodes' trr can > be horrendously slow--at least once upon a time on power FETs they > often were... >Datasheets aren't what they used to be, lots of specs lacking. The question that needs to be pondered though is, does trr really matter in Klaus' case? RS485 isn't exactly like a Maserati so even if ringing pings the body diodes a little it probably won't matter. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●December 22, 20122012-12-22
Klaus Kragelund wrote:> On Saturday, December 22, 2012 7:43:22 AM UTC+1, miso wrote: >> If supply voltage is the only issue, have you investigated using a DC/DC >> >> plus an off the shelf 485? > > Yes, currently we are using off the shelf RS485, but the bus voltage is high due to 3V supply, so the power budget is exceeded > > We have seen no devices below 3V that can drive 54 ohms >There could be hope: http://www.electronicsweekly.com/Articles/13/07/2012/54115/intersil-has-rs-485-transceiver-running-off-1.8v.htm Maybe give them a ring but then I'd also check for availability. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●December 22, 20122012-12-22
On Friday, December 21, 2012 3:02:40 AM UTC-8, Klaus Kragelund wrote:> The standard RS485 drivers available has a minimum voltage of 3V and a rarther large drop voltage when loaded with the defined bus load ... > So, we are thinking about designing our own driver in discrete components, so we can reduce the supply down to 2V and still comply with minimum 1.5V differential voltage into 54ohms.RS-485 is a multidrop bus, with termination resistors; do you control the termination resistors, or are they external to your design? Your 'supply down to 2V' circuitry will have to be capable of safely handling the RS-485 bus signal range, -7V to +12V, or you can discard the standard entirely, and use something more appropriate to low-power implementation.
Reply by ●December 22, 20122012-12-22
On Saturday, December 22, 2012 3:51:27 PM UTC+1, Lasse Langwadt Christensen= wrote:> On Dec 21, 8:11=A0pm, Klaus Kragelund <klausk...@hotmail.com> wrote: >=20 > > On Friday, December 21, 2012 6:16:02 PM UTC+1, John Larkin wrote: >=20 > > > On Fri, 21 Dec 2012 03:02:40 -0800 (PST), Klaus Kragelund >=20 > > >=20 > > > <klausk...@hotmail.com> wrote: >=20 > > >=20 > > > >Hi >=20 > > >=20 > > > >The standard RS485 drivers available has a minimum voltage of 3V and=a rarther large drop voltage when loaded with the defined bus load for Mod= bus of 54ohms, and this causes problems for our design since we have limite= d power available for driving the bus>=20 > > >=20 > > > >So, we are thinking about designing our own driver in discrete compo=nents, so we can reduce the supply down to 2V and still comply with minimum= 1.5V differential voltage into 54ohms.>=20 > > >=20 > > > >We only need 115k baud, so we could use a tiny logic level FET as th=e output stage. Shortcircuit protection would be done with a current limit = circuit along with a low value supply capacitance (to reduce peak power in = the FETs)>=20 > > >=20 > > > >Backfeed would need to be solved with a beefy diode to a defined cla=mp voltage.>=20 > > >=20 > > > >So, anyone been down this road, designing your own RS485 driver? >=20 > > >=20 > > > >Cheers >=20 > > >=20 > > > >Klaus >=20 > > >=20 > > > Just use a cmos quad xor gate; two paralleled sections for one phase,=two for>=20 > > >=20 > > > the other, with maybe 3.3 volt supply and 30 ohm source terminations.=There's no>=20 > > >=20 > > > need to use discrete fets. >=20 > > >=20 > > > We recently did this: >=20 > > >=20 > > >https://dl.dropbox.com/u/53724080/Circuits/ESM/Line_Drivers.pdf >=20 > > >=20 > > > The basic line driver is a couple of tiny-logic gates driven from com=plementary>=20 > > >=20 > > > FPGA outputs. The downstream junk is selectable line driver equalizat=ion, to>=20 > > >=20 > > > partially correct for CAT5 cable losses. This runs up to 125 MHz. >=20 > > >=20 > > Maybe a good point, if I can find a logic device that has low RDSon at =2V.>=20 > > >=20 > > The ones I have found have 10ohms RDSon (NC7SZ74), but could parallel s=ome of those to bring down the RDSon to the 2-3 ohms range>=20 > > >=20 > > Regards >=20 > > >=20 > > Klaus >=20 >=20 >=20 > how about something like this? >=20 >=20 >=20 > http://www.analog.com/static/imported-files/data_sheets/ADG811_812_813.pd=f>=20 >=20 >=20 > -LasseYes, I have been down that road. The one you refer to here, has very good l= atchup properties (over 300mA), and that sure beats what I have found sofar= :-) Regards Klaus
Reply by ●December 22, 20122012-12-22
On Sat, 22 Dec 2012 09:08:51 -0800, Joerg <invalid@invalid.invalid> wrote:>Klaus Kragelund wrote: >> On Saturday, December 22, 2012 7:43:22 AM UTC+1, miso wrote: >>> If supply voltage is the only issue, have you investigated using a DC/DC >>> >>> plus an off the shelf 485? >> >> Yes, currently we are using off the shelf RS485, but the bus voltage is high due to 3V supply, so the power budget is exceeded >> >> We have seen no devices below 3V that can drive 54 ohms >> > >There could be hope: > >http://www.electronicsweekly.com/Articles/13/07/2012/54115/intersil-has-rs-485-transceiver-running-off-1.8v.htm > >Maybe give them a ring but then I'd also check for availability.I must say, I am really impressed by those specs. However, you have to play the Devil's advocate to select a usable product. A quick look suggests that you have to use at least Vcc > 3.0 V to be within RS-485 common mode range. I have not looked at other parameters.
