Comparison of RS-485 transceivers
Implementing a RS-485 can be addressed in many ways depending upon different factors such as reliability, cost or component part number. Before going through a target implementation, it is necessary bring a few important concepts to the table.
RS-485 important concepts
Differential signaling
RS-485 has a differential signaling level. A twisted pair consist of three copper parts:
- V+
- V-
- Shield
If V+ - V- is greater than 200mV the receiver interprets the differential input as a logic high. When V+ - V- is less than -200mV, the receiver interprets the differential input as a logic low. Anything in between is a not stable state. That is called a fail operation.
Special attention should be paid to the shielding, which will carry back the common-mode current
Terminations
Several terminations can be used:
- Parallel
- RC
- Fail-safe: it corresponds to a Thevenin termination in which the voltage is the supply voltage of the circuit.
Fail-safe operation
There are times in which the line is not driven or the quality of the differential signal is unacceptable. In that cases we have an open-circuit or the voltage is lower than +- 200mV, respectively. A good solution to these faulty situations can be found in a fail-safe termination in which the input is forced to a high logic state.
Reducing consumption in specific uses
An user can deliberately decide not to drive a RS-485 interface, resulting in a fail operation in the receiver if no fail-safe termination in used. Side-effects of the shutdown correspond to a delay in the transmitter and a dead receiver. However, reducing the consumption of the link can be obtained without damaging the functionality. A case is presented below:
> Let's consider a link in which the specifications are as follows:
> * Low transmission rate.
> * Running for long periods of time.
> * Delay-tolerant.
>
> Reducing the consuption in the driver renders into an EXCELLENT
solution!
Isolation
RS-485 is common-mode voltage robust by its inception: it can handle voltages in any of its V+ and V- pins in the range of -7V to 12V. Indeed there are ICs which can go beyond that range, i.e. -20V to 20V, without the need of a galvanic isolation.
For high-reliability links in which long-distance and eventual high voltage-trasients are important factors to be taken into account, galvanic isolation helps the designer to provide the required reliability. Three different schemas can be used to design a galvanically isolated rs-485 system:
- Placing an optocoupler in the TTL side of the RS-485 module and an isolated power supply to provide power pins to the optocoupler and the receiver. That means three different ICs.
- Integrated complete isolation of the RS-485 module. Only one IC is
needed:
- An isolated RS-485 transceiver
- An isolated termination voltage for fail-safe termination.
Reception thresholds
An important feature not to be forgotten are reception thresholds. By selecting a device with asymmetric thresholds we can detect no signal in a link. The key idea behind this is to select the thresholds so that the hysteris window it is not overlapping with itself negated. Basically it means that both low and high threshold must have the same sign.
A complete explanation can be found in Texas Instruments' Application Note SLYT257
Comparison table of RS-485 transceivers
Non-isolated transceivers
Manufacturer | Product | Price | Supplier |
---|---|---|---|
Analog Devices Inc. | ADM1485ARZ | 1.7796 € | Newark |
Analog Devices Inc. | ADM4852ARZ | 1.08972 € | Digi-Key |
Maxim-IC | MAX3440EESA | 2.677 € | Digi-Key |
Linear Technologies | LTC1785CN | 2.30 USD | Arrow |
Texas Instruments | SN65HVD20D | 2,38 € | Farnell |
Non-isolated transceivers, equalized
Texas Instruments | SN65HVD24D | 3,34 € | Farnell |
---|
Isolated transceivers
Manufacturer | Product | Price | Supplier |
---|---|---|---|
Analog Devices Inc. | ADM2483BRWZ | 4.4623 € | Digi-Key |
Linear Technologies | LTC1535CSW | 6.35 USD | Arrow |
Analog Devices Inc. | ADM2490EBRWZ | 6.6788 € | Digi-Key |
Analog Devices Inc. | ADM2486BRWZ | 6.8018 € | Digi-Key |
Linear Technologies | LTC1535ISW | 7.30 USD | Arrow |
Analog Devices Inc. | ADM2485BRWZ | 7.517 € | Digi-Key |
Texas Instruments | ISO1176TDW | 7,67 € | Farnell |
Texas Instruments | ISO3086TDW | 8,06 € | Farnell |
Complete isolated transceivers
Manufacturer | Product | Price | Supplier |
---|---|---|---|
Analog Devices Inc. | ADM2587EBRWZ-ND | 7.589 € | Digi-Key |
Analog Devices Inc. | ADM2582EBRWZ | 9.91 USD | Arrow |
Analog Devices Inc. | ADM2682EBRIZ | 11.41 USD | Arrow |
Analog Devices Inc. | ADM2587EBRWZ | 12.01 € | Farnell |
Linear Technologies | LTM2881CV | 12.07 USD | Arrow |
Linear Technologies | LTM2881CY | 12.07 USD | Arrow |
Linear Technologies | LTM2881HY | 12.7963 € | Digi-Key |
Linear Technologies | LTM2881HY | 12.7963 € | Digi-Key |
Maxim-IC | MAX1480ACPI+ | 18.32 AUD | Mouser |
Comparison of RS'485 receivers (only)
Here can be found a comparison of transceivers selected because its receptor has asymmetrical thresholds that allow to perform no-singal detection in a link.
Manufacturer | Product | Price | Supplier | * Pin-to-pin compatible group[] |
---|---|---|---|---|
Texas Instruments | SN65HVD1782D | 3,87 € | Farnell | A |
Texas Instruments | SN65HVD3088EDG4 | 1,51 € | Farnell | A |
References
National Instruments
- Application Note AN-409
- On what RS-485 is, and what adds to RS-422
- Application Note AN-759
- Multipoint Applications. It is interesting for considering situations that can eventually happen due to strange interconnections.
- Application Note AN-903
- Differential Termination Techniques.
- Application Note
AN-1031
- Good overview to RS-422.
- Application Note
AN-1057
- Ten Ways to Bulletproof RS-485 Interfaces
Texas Instruments
- Application Note
SLLA272B
- Guidelines for RS-485 link design
- Application Note SLLA322
- Comparison of different transceivers in terms of noise immunity. TI's ones won.
- Application Note SLLA292A
- Protection RS-485 Interfaces and a brief introduction to Transient Immunity Tests.
- Application Note SLLA169 and
some good
slides
- Receiver equalization techniques in RS-485
- Application Note
SLYT257
- By using two passive failsafe receivers, it shows how to detect if a link is alive.
- Application Note
SNLA031
- Instead of notifying a failsafe condition, by carefully selecting pulling resistors a default failsafe input can be set.
- Application Note
SLYT064
- Different approaches and products are offered for failsafe operation.
[*] Pin-to-pin compatible group: it means that every IC assigned
with a group identifier can be seamlessly interchanged within the same
group.
---
Carlos Gil Soriano - 24-01-2012