Hi 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 Modbus of 54ohms, and this causes problems for our design since we have limited power available for driving the bus 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. We only need 115k baud, so we could use a tiny logic level FET as the 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) Backfeed would need to be solved with a beefy diode to a defined clamp voltage. So, anyone been down this road, designing your own RS485 driver? Cheers Klaus

# Discrete custom design of RS485 driver

Started by ●December 21, 2012

Reply by ●December 21, 20122012-12-21

On Friday, December 21, 2012 12:02:40 PM UTC+1, Klaus Kragelund wrote:> Hi >=20 >=20 >=20 > The standard RS485 drivers available has a minimum voltage of 3V and a ra=rther large drop voltage when loaded with the defined bus load for Modbus o= f 54ohms, and this causes problems for our design since we have limited pow= er available for driving the bus>=20 >=20 >=20 > 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.>=20 >=20 >=20 > We only need 115k baud, so we could use a tiny logic level FET as the out=put stage. Shortcircuit protection would be done with a current limit circu= it along with a low value supply capacitance (to reduce peak power in the F= ETs)>=20 >=20 >=20 > Backfeed would need to be solved with a beefy diode to a defined clamp vo=ltage.>=20 >=20 >=20 > So, anyone been down this road, designing your own RS485 driver? >=20A rough first draft: www.electronicsdesign.dk/tmp/RS485_Custom.pdf

Reply by ●December 21, 20122012-12-21

On Fri, 21 Dec 2012 04:27:55 -0800 (PST), Klaus Kragelund <klauskvik@hotmail.com> wrote:>On Friday, December 21, 2012 12:02:40 PM UTC+1, Klaus Kragelund wrote: >> Hi >> >> >> >> 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 Modbus of 54ohms, and this causes problems for our design since we have limited power available for driving the bus >> >> >> >> 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. >> >> >> >> We only need 115k baud, so we could use a tiny logic level FET as the 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) >> >> >> >> Backfeed would need to be solved with a beefy diode to a defined clamp voltage. >> >> >> >> So, anyone been down this road, designing your own RS485 driver? >> > >A rough first draft: > >www.electronicsdesign.dk/tmp/RS485_Custom.pdfWith only a 2V supply, how do you get enough drive for the P-channel device? Do you have a more negative supply available? An amusing thought... I know that open drain output logic exists, at least with an N-channel output. Does such a thing exist with a P-channel open-drain? ...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.

Reply by ●December 21, 20122012-12-21

On Fri, 21 Dec 2012 03:02:40 -0800 (PST), Klaus Kragelund <klauskvik@hotmail.com> 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 for Modbus of 54ohms, and this causes problems for our design since we have limited power available for driving the busThis is not a Modbus specific issue, but rather RS-485 specific issue with a twisted pair bus with characteristic impedance of 100-120 ohms. In order to avoid reflections at the open ends of the bus cable, termination resistors are typically used at both ends with the same value as the cable characteristic impedance. For DC, those two resistors are effectively in parallel and hence the 45 ohm total load. However, those termination resistors are needed only to avoid the reflections from voltage _transitions_. Thus, putting a capacitor in series with the termination resistor(s) should reduce the idle power consumption, when no data is being sent. Of course, without DC continuity, the end to end signal ground conductor is essential. There are application notes describing even more elaborate termination methods, describing their advantages and disadvantages. You should also look for various termination techniques used on CAN bus (which is essentially RS-485).

Reply by ●December 21, 20122012-12-21

On Fri, 21 Dec 2012 03:02:40 -0800 (PST), Klaus Kragelund <klauskvik@hotmail.com> wrote:>Hi > >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 Modbus of 54ohms, and this causes problems for our design since we have limited power available for driving the bus > >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. > >We only need 115k baud, so we could use a tiny logic level FET as the 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) > >Backfeed would need to be solved with a beefy diode to a defined clamp voltage. > >So, anyone been down this road, designing your own RS485 driver? > >Cheers > >KlausJust use a cmos quad xor gate; two paralleled sections for one phase, two for the other, with maybe 3.3 volt supply and 30 ohm source terminations. There's no need to use discrete fets. We recently did this: https://dl.dropbox.com/u/53724080/Circuits/ESM/Line_Drivers.pdf The basic line driver is a couple of tiny-logic gates driven from complementary FPGA outputs. The downstream junk is selectable line driver equalization, to partially correct for CAT5 cable losses. This runs up to 125 MHz. -- 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 21, 20122012-12-21

Klaus Kragelund wrote:> On Friday, December 21, 2012 12:02:40 PM UTC+1, Klaus Kragelund > wrote: >> Hi >> >> >> >> 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 Modbus of 54ohms, and this causes problems for our design >> since we have limited power available for driving the bus >> >> >> >> 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. >> >> >> >> We only need 115k baud, so we could use a tiny logic level FET as >> the 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) >> >> >> >> Backfeed would need to be solved with a beefy diode to a defined >> clamp voltage. >> >> >> >> So, anyone been down this road, designing your own RS485 driver? >> > > A rough first draft: > > www.electronicsdesign.dk/tmp/RS485_Custom.pdfJust a comment: Diodes are already in the FETs, in the form of body diodes. One thought would be whether a hysteretic sync-buck IC could be pressed into service here. I haven't needed one this low in voltage yet but they should come for very low supply voltages (processor core supplies and such). -- Regards, Joerg http://www.analogconsultants.com/

Reply by ●December 21, 20122012-12-21

On Friday, December 21, 2012 5:09:01 PM UTC+1, Jim Thompson wrote:> On Fri, 21 Dec 2012 04:27:55 -0800 (PST), Klaus Kragelund >=20 > <klauskvik@hotmail.com> wrote: >=20 >=20 >=20 > >On Friday, December 21, 2012 12:02:40 PM UTC+1, Klaus Kragelund wrote: >=20 > >> Hi >=20 > >>=20 >=20 > >>=20 >=20 > >>=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 Modbu= s of 54ohms, and this causes problems for our design since we have limited = power available for driving the bus>=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> So, we are thinking about designing our own driver in discrete compone=nts, so we can reduce the supply down to 2V and still comply with minimum 1= .5V differential voltage into 54ohms.>=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> We only need 115k baud, so we could use a tiny logic level FET as the =output stage. Shortcircuit protection would be done with a current limit ci= rcuit along with a low value supply capacitance (to reduce peak power in th= e FETs)>=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> Backfeed would need to be solved with a beefy diode to a defined clamp=voltage.>=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> So, anyone been down this road, designing your own RS485 driver? >=20 > >>=20 >=20 > > >=20 > >A rough first draft: >=20 > > >=20 > >www.electronicsdesign.dk/tmp/RS485_Custom.pdf >=20 >=20 >=20 > With only a 2V supply, how do you get enough drive for the P-channel >=20 > device? Do you have a more negative supply available? >=20The voltage rail for the FET are driven by 2V and I will generate an additi= onal supply voltage to drive the gates, about 3V. The majority of the power goes for the bus, driving the 54ohms load (120//1= 20//1500 ohms in parallel, that is two termination resistors and the 32 uni= t load impedance).=20 Right now the implementation is using a standard RS485 driver running at 3V= supply, but with 54 ohms resistance along with the driver impedance, draws= 90mW during transmission. A low RDSon driver at 2V would reduce that to about 60mW Regards Klaus

Reply by ●December 21, 20122012-12-21

On Friday, December 21, 2012 7:12:56 PM UTC+1, Joerg wrote:> Klaus Kragelund wrote: > > > On Friday, December 21, 2012 12:02:40 PM UTC+1, Klaus Kragelund > > > wrote: > > >> Hi > > >> > > >> > > >> > > >> 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 Modbus of 54ohms, and this causes problems for our design > > >> since we have limited power available for driving the bus > > >> > > >> > > >> > > >> 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. > > >> > > >> > > >> > > >> We only need 115k baud, so we could use a tiny logic level FET as > > >> the 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) > > >> > > >> > > >> > > >> Backfeed would need to be solved with a beefy diode to a defined > > >> clamp voltage. > > >> > > >> > > >> > > >> So, anyone been down this road, designing your own RS485 driver? > > >> > > > > > > A rough first draft: > > > > > > www.electronicsdesign.dk/tmp/RS485_Custom.pdf > > > > > > 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. Regards Klaus

Reply by ●December 21, 20122012-12-21

On Friday, December 21, 2012 5:57:52 PM UTC+1, upsid...@downunder.com wrote= :> On Fri, 21 Dec 2012 03:02:40 -0800 (PST), Klaus Kragelund >=20 > <klauskvik@hotmail.com> wrote: >=20 >=20 >=20 > >The standard RS485 drivers available has a minimum voltage of 3V and a r=arther large drop voltage when loaded with the defined bus load for Modbus = of 54ohms, and this causes problems for our design since we have limited po= wer available for driving the bus>=20 >=20 >=20 > This is not a Modbus specific issue, but rather RS-485 specific issue >=20 > with a twisted pair bus with characteristic impedance of 100-120 ohms. >=20 > In order to avoid reflections at the open ends of the bus cable, >=20 > termination resistors are typically used at both ends with the same >=20 > value as the cable characteristic impedance.=20 >=20 >=20 >=20 > For DC, those two resistors are effectively in parallel and hence the >=20 > 45 ohm total load. >=20 >=20 >=20 > However, those termination resistors are needed only to avoid the >=20 > reflections from voltage _transitions_. Thus, putting a capacitor in >=20 > series with the termination resistor(s) should reduce the idle power >=20 > consumption, when no data is being sent. Of course, without DC >=20 > continuity, the end to end signal ground conductor is essential. >=20 >=20 >=20 > There are application notes describing even more elaborate termination >=20 > methods, describing their advantages and disadvantages. You should >=20 > also look for various termination techniques used on CAN bus (which is >=20 > essentially RS-485).Yes, but to conform to the Modbus standard, the termination resistors are a= dded without diodes Cheers Klaus

Reply by ●December 21, 20122012-12-21

On Friday, December 21, 2012 6:16:02 PM UTC+1, John Larkin wrote:> On Fri, 21 Dec 2012 03:02:40 -0800 (PST), Klaus Kragelund >=20 > <klauskvik@hotmail.com> wrote: >=20 >=20 >=20 > >Hi >=20 > > >=20 > >The standard RS485 drivers available has a minimum voltage of 3V and a r=arther large drop voltage when loaded with the defined bus load for Modbus = of 54ohms, and this causes problems for our design since we have limited po= wer available for driving the bus>=20 > > >=20 > >So, we are thinking about designing our own driver in discrete component=s, so we can reduce the supply down to 2V and still comply with minimum 1.5= V differential voltage into 54ohms.>=20 > > >=20 > >We only need 115k baud, so we could use a tiny logic level FET as the ou=tput stage. Shortcircuit protection would be done with a current limit circ= uit 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 clamp v=oltage.>=20 > > >=20 > >So, anyone been down this road, designing your own RS485 driver? >=20 > > >=20 > >Cheers >=20 > > >=20 > >Klaus >=20 >=20 >=20 >=20 >=20 > Just use a cmos quad xor gate; two paralleled sections for one phase, two=for>=20 > the other, with maybe 3.3 volt supply and 30 ohm source terminations. The=re's no>=20 > need to use discrete fets. >=20 >=20 >=20 > We recently did this: >=20 >=20 >=20 > https://dl.dropbox.com/u/53724080/Circuits/ESM/Line_Drivers.pdf >=20 >=20 >=20 > The basic line driver is a couple of tiny-logic gates driven from complem=entary>=20 > FPGA outputs. The downstream junk is selectable line driver equalization,=to>=20 > partially correct for CAT5 cable losses. This runs up to 125 MHz.Maybe a good point, if I can find a logic device that has low RDSon at 2V.= =20 The ones I have found have 10ohms RDSon (NC7SZ74), but could parallel some = of those to bring down the RDSon to the 2-3 ohms range Regards Klaus