The standard defines minimum 1.5V into 54ohms. I need to comply to that to be caompatible to other devices. The difference from 1.5V to 200mV is the noise and attenuation margin.
Modbus defines 1200m maximum length
We cannot know if we are in a multidrop system or if the user uses a point-to-point system (1 device). So the duty cycle can approach 50% and to avoid storage capacitors we would need to supply the full TX current (low baud rate constraint)
We have failsafe resistors, to put the bus in known state if the wires are open circuit
We use the 3.5c to our advantage, since that prolongs the low current RX state
Modbus is funny, the standard has contracteditions, and the industry users has invented their own semistandard. We have scanned the usergroups to be sure we cover the "funny" cases
We have not found any drivers below 3V supply that can drive 54 ohms, and that
nd a rarther large drop voltage when loaded with the defined bus load for M odbus of 54ohms, and this causes problems for our design since we have limi ted power available for driving the bus
ponents, so we can reduce the supply down to 2V and still comply with minim um 1.5V differential voltage into 54ohms.
the output stage. Shortcircuit protection would be done with a current limi t circuit along with a low value supply capacitance (to reduce peak power i n the FETs)
lamp voltage.
e, two for
s. There's no
omplementary
ation, to
t 2V.
some of those to bring down the RDSon to the 2-3 ohms range
plex.
Our device is one out of many Modbus device from other suppliers, so we can not do other than comply to RS485/Modbus standard
That referenced Modbus.org document states on page 27,
I am fully aware that the RS-485 specifies maximum 1200 m or to be pedantic 4000 ft.
The Modbus was developed by Modicon to be used in their PLCs in the
1970's. By studying the various Modicon PLC documentations, there were already some differences. When other vendors tried to create compatible products, the number of variations grew significantly.
By creating the Modbus.org organization, there was an attempt to standardize the situation to ensure interoperability.
I still do not understand, why that referenced document uses conventions different from the RS-485 standard, such as 1000 m vs.
1200 m. or why they call the serial lines D0 and D1 instead of RS/EIA/TIA-485 lines A and B.
To make situation worse, some manufacturers call those lines D+ and D- and different manufacturers can't even decide which is Plus and which is Minus, so you may have to connect D+ from one vendor to D- of an other manufacturer and vice versa. One manufacturer uses the driver chip manufacturer convention, while other use established practices:-).
The original Modbus protocol only specified binaries (coil/binary input) and 16 bit scalars (register/input registers), Since there are needs to transfer 32 bit integers and 32 bit IEEE floats, most manufacturers have elected to use consecutive 16 bit registers to carry those beasts and the receiver then has to concatenate these two registers into a single value. Unfortunately, one manufacturer puts the most significant part into the lower numbered register, while the other into the higher numbered register :-).
If this would not be enough to create confusion, some "Modbus compatible" device when reading registers 5001 to 5002 might return 8 bytes (two 32 long ints/floats). To read two registers at 1001 will return 4 bytes (two 16 bit registers).
There are enough interoperability problems without adding your own ad hoc serial line drivers :-). For interoperability, it might be better to solve the power issues than trying to invent something special of your own.
FYI, note their switching speed measurements specify "RG=0", which really just means the figures they give are utterly meaningless.
My experience is, t_rr is roughly proportional to Vds(max), and usually about 2-3 times the rating of the fastest junction diode of comparable ratings.
Example:
- The above does 60V and 100A, and the body diode is specified at 25A. There aren't any junction diodes available to compare with that, they're all schottky. (A plain old UF4001 is rated 50V, 1A, and 50ns, but it's in the same bracket as UF4004, which ought to be 50ns. A true, optimized UF4001 should be quite fast indeed, basically 3-4 x 1N914 in parallel.)
- A better comparison would be made between, say, 200V, 600V and 1200V devices. Offhand, I think the ratings for something like a 200V, 10A MOSFET will be maybe 100ns, and a 200V, 10A diode, 50ns; at 600V 10A, more like 250ns and 100ns; and at 1200V 10A, around 350ns and 150ns.
Tim
--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com
In the documents from Modbus.org they have contradictions in the same document. A lot of weak statements like "shall" and possibility to add power on the RJ45 connector, which seems almost non-existant in the industry
We have observed the same. We have a setup where we have tried different modules and in some cases we need to switch A and B
We are new to this, so we have leaned against an experienced consultanty house, to use the sensible modbus function codes. We discussed the broadcast code, but did not implement it since no error checking is possible (no response)
During our tech scan we found a common PLC, that even did not comply to 3.5c. Users simply made workarounds for that, since it was from a major supplier.
We will design it to comply with the standard, supplying 1.5V during TX and
200mV trigger level during RX. We will simply use better (lower RDSon) transistors to allow for lower supply voltage.
The receiving circuit could be done just by a standard IC (only with RX functionality), so we don't need to worry to much about that part of the design
nd a rarther large drop voltage when loaded with the defined bus load for M odbus of 54ohms, and this causes problems for our design since we have limi ted power available for driving the bus
ponents, so we can reduce the supply down to 2V and still comply with minim um 1.5V differential voltage into 54ohms.
the output stage. Shortcircuit protection would be done with a current limi t circuit along with a low value supply capacitance (to reduce peak power i n the FETs)
lamp voltage.
e, two for
s. There's no
omplementary
ation, to
t 2V.
some of those to bring down the RDSon to the 2-3 ohms range
plex.
I am afraid not, LVDS has to low voltage levels and common mode compliance to conform to RS485
Excellent, that is what I have recommended for my customers for decades (and most adhere to this recommendation :-).
However, since you do not have full control of the network, there might still be large potential differences between signal line(s), signal ground and protective ground.
Now, putting my system integrator hat on :-)
I am trying not to be un polite, but while it is a bad thing if a point-to-point connection fails due to driver latchups, it is _completely_unacceptable_ if such problems will stall the traffic on a multivendor RS-485 bus !!
Please consider twice, before you are going to implement your own discrete driver designs. As a system integrator, I would promptly throw out such designs, or at least allocate an own serial line (with extra cost) for such devices.
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
nts, 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 ci rcuit along with a low value supply capacitance (to reduce peak power in th e FETs)
voltage.
wo for
here's no
ementary
n, to
.
e of those to bring down the RDSon to the 2-3 ohms range
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.
I 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.
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.
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.
Social media is the next bubble that will go *KABLAM* on the financial markets.
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.
large drop voltage when loaded with the defined bus load ...
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.
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
f
Yes, 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 :-)
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.