a rarther large drop voltage when loaded with the defined bus load for Modb us of 54ohms, and this causes problems for our design since we have limited power available for driving the bus
ents, so we can reduce the supply down to 2V and still comply with minimum
1.5V differential voltage into 54ohms.
output stage. Shortcircuit protection would be done with a current limit c ircuit along with a low value supply capacitance (to reduce peak power in t he FETs)
p voltage.
two for
There's no
lementary
on, to
V.
me of those to bring down the RDSon to the 2-3 ohms range
Yes, I need to tristate the driver, since it is a 2 wire system, half duple x.
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
nents, so we can reduce the supply down to 2V and still comply with minimum 1.5V differential voltage into 54ohms.
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)
mp voltage.
two for
There's no
plementary
ion, to
2V.
ome of those to bring down the RDSon to the 2-3 ohms range
We would need to add protection in any case to reduce the currents, the dri ver IC would suffer from latchup problems if not.
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
components, so we can reduce the supply down to 2V and still comply with minimum
1.5V differential voltage into 54ohms.
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)
voltage.
for
There's no
complementary
to
of those to bring down the RDSon to the 2-3 ohms range
IC would suffer from latchup problems if not.
I've never seen a cmos chip latch up from driving a low-z load, including a short to either rail. You can get them pretty hot. Pushing a lot of current into an ESD diode can latch some parts. We use tiny logic drivers in the thing I posted, and in other products that we've sold thousands of, driving customer loads of all sorts, and haven't had any latchup problems.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
But resistive, since it's a current path from the back of the die.
Go ahead... I always enjoy your sound effects >:-}
I mis-spoke... make that Synaptics.
Here today, China tomorrow ;-) ...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.
And that's not even that big of a deal, really -- if you run 2N4401/3 kind of hot (~20mA Ic, 2mA base drive, 680 ohm B-E resistor), you'll see edges under 100ns and storage time under 300ns (storage causes skew, but it's symmetrical in an H-bridge, so it causes shoot-through and delay).
115kbaud gives you almost 10us between edges, so there's tons of time for switching.
The average switching transistor (like the little complementary gate drive things, or just a plain old ZTX651 or etc.) is even beefier, maintaining hFE > 100 at rated collector current. So, even saturated (where hFE is lower and stored charge piles up), they don't take much drive current at all, relative to what they're doing. They start looking like low Vgs(th) MOSFETs, with an input diode thrown in for convenience.
Tim
--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com
What's the difference? Whether 0.7V drops across a more or less resistive path, who cares? All that counts is total dissipation and that it's not too localized.
I sure hope we can at least keep high-end semiconductor engineering and processing in the country for a while, now that Obamacare is smothering much of the med device investment climate. We can't afford to lose such leadership positions but obviously that doesn't sink in on the hill :-(
Except for the expert witness stuff (3 weeks in Dallas Federal District Court starting around January 7 :-), all of my work is now west of Honolulu. ...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.
Many reasons. A major one is the new medical device tax. This will skim
2.3% off the top. From revenue. It means that start-up companies that typically do not turn a profit for easily the first 10 years will get burdened with it. For others that are in the disposables business and run on 5-10% profit margins one can easily see that it will wipe out a huge chunk of that.
Then there's rationing that'll happen. Medical is a zero sum game, we can't run it on hot air (a.k.a. bonds) forever. We have already seen slashing of reimbursements. This will make certain higher end products unprofitable, and high-end is where our country excels. So while now everyone may be entitled to a free enema or flu shot there are likely going to be growing waiting lists when you need help with some serious stuff. Just like there are in Canada. This means less revenue for manufacturers.
Next, capital gains taxes. Funding a start-up is a major risk for the investors. So if the rewards they can reap are majorly cut down by tax increases they will, obviously, curb their risk exposure. That's what I am seeing the most in the med venture space, a risk-averseness that I have not encountered ever in the past. And I am in med tech since the 80's.
I could go on and on, but hopefully this suffices to explain some of the effects that I (and a lot of others) see coming or have already experienced. Personally I've seen things not being funded that used to be a sure thing 10 years ago, plus whole projects got canned that had progressed quite far. While people like me can move to other areas in engineering because I have kept a generalist attitude all my life that is no so for many others. My former employer is in the process of moving production to Costa Rica. Other companies have started layoffs:
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
components, so we can reduce the supply down to 2V and still comply with minimum
1.5V differential voltage into 54ohms.
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)
voltage.
for
There's no
complementary
to
of those to bring down the RDSon to the 2-3 ohms range
Can you make the whole system LVDS?
--
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
driver IC would suffer from latchup problems if not.
Usually the latchup current specs are considerably larger than the possible output currents. Therefore, neither driver nor receiver should be able to latch from reflections.
Lightning's a different matter.
We fielded some RTUs ages ago, a new design where I tried a number of ideas, including improved lightening protection. Annual field failures went down from numerous to two, one of which had a several- inch lightning hole burned thru the heavy-gauge steel enclosure, a direct hit.
Is it possible the VC is going to stupid internet social media garbage? Ya think?
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.
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.
The power dissipation issue is worse in point-to-point RS-422 connections, in which the transmitters are constantly on and must be able to supply 2 mA to the termination resistor at the opposite end, even when the line is idle.
However, Modbus over RS-485 is a half duplex protocol with at most one transmitter in the active state feeding the termination, while all the other are in the tri-state. Thus the total system consumption is quite low. Due to the request/response nature of Modbus, the master Tx duty cycle would be around 50 %, while in a multidrop system, the duty cycle for the Tx is much lower, perhaps a few percent, keeping the total energy consumed at a low value for each device.
By using 120 /120 /1500 ohms, it appears that you do not intend to use polarization a.k.a pullup/pulldown resistors. While no data is being transmitted, no transmitter is active the bus in a high impedance state, with only the receiver leakages as a load. That 1500 ohms was for 32 slaves, but with only a few slaves, the impedance is quite high and sensitive to electrostatically connected interference (typically false start bits).
In Modbus standard, this has been accounted for, by requiring that the pause between individual bytes must not be greater than 1.5 character times and that when the transmitter is turned from tri-state to active, it must send the idle (Mark, "1") state for at least 3.5 character times. This will allow any reflections to die out and during this period, any receiver will flush any spurious line noise (the 1.5 character time rule). Then the actual message transmission is performed, followed by the Tx driven actively to idle (Mark, "1") state for an additional 3.5 character times. This allows reliable end of frame detection, by suppressing any reflections etc. immediately after the data frame.
However, in a multivendor environment, it is not clear how well these timing rules are followed, especially at high speeds, thus to ensure maximum interoperability, I would definitively recommend using those pullup/pulldon resistors on the bus to lower the impedance levels while all transmitters are in the tri-state.
Regarding the 1 nF+120 ohm termination, the RC time constant is 0.12 us, hence after a few RC time constants, say 0.5 us after the last signal transition, practically no power is delivered to the termination resistance. At 115k2 with bit times of 9 us, power is dissipated during 6 % of the bit time. Since there can be consecutive "00..." or "11..." sequences in a message, without state changes between bits (NRZ), the actual duty cycle is even less.
So I really do not understand, why you want to design a custom driver to reduce power dissipation.
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.
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.
Thanks for such an informative post - I have been using/implementing modbus, on occasion, for 15 years. And have never seen this stuff spelled out before.
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