Isolated RS-232 with Integrated Signal and Power Reference Design
Description
This TI Design supplies a compact solution able to generating isolated DC power while supporting isolated RS-232 communication. The TIDA-01230 consists of a reinforced digital isolator with integrated power combined with an RS-232 communication module. The solution is designed in a way regarding occupy minimal place to achieve this solution.
Features
- Smallest Combination Solution (14.8 mm × 11.5 mm):
– Slightly Bigger than ISOW7842 Device Footprint
- Single Input Power Solution:
– No Separate Isolated Supply Required to Power Interface-Side Components
- Reduced BOM Cost; Solution Eliminates Requirement of Separate Transformer
- Simpler Design
- Extendable to Other RS-232 Transceivers
Applications
o Grid Communication Modules
o Building Automation and HVAC
o Industrial Automation
o Remote Radio Unit (RRU) and Base Band Unit (BBU)
o Medical Equipment
o Other Communication Modules Coping with High-Voltage Systems
1 System Overview
1.1 Design Theory
1.1.1 Isolation in Data Communication
Most industrial interface systems are places that noise can seriously affect the integrity of data transfer. A tested method of improving noise performance for just about any interface circuit is galvanic isolation. Isolating the low-voltage (LV) microcontroller (MCU) side in the high-voltage (HV) interface side has shown to become an effective means to fix protect from data corruption and to protect LV side components from seeing HV stresses like electrostatic discharge (ESD), electrical fast transient (EFT), and surge.
Common-mode noise effects and many types of radiated noise can impact the communication system.
Unwanted currents and voltages on the cable bus connecting multiple systems can cause severe problems.
These voltages and currents come primarily from two sources: ground loops and electrical line surges. Ground loops occur whenever a bus or system uses multiple ground paths. Two system grounds attached to the bus and separated by hundreds or thousands of meters may not have exactly the same potential. Because of this potential difference, current flows between these points. This unintended current flow can harm or destroy components. Electrical surges could be brought on by many sources, caused by currents coupled onto cable lines through induction. Long cable lines in industrial environments are specifically prone to this phenomenon.
The operation of electrical motors, particularly, causes rapid alterations in the ground potential. These changes can generate a current flow through your regional lines to equalize the floor potential. Other HV stress voltage sources include ESDs, coupled transient noises (EFT) and lightning strikes (surge). These induced surges can lead to a large number of volts of potential on the line, and manifest themselves as transient current and voltage surges. Thus, an online node may receive a 5- V switching signal superimposed with an HV level with regards to the local ground. These uncontrolled voltages and currents can corrupt the signal and become catastrophic towards the device and system, damaging or destroying the constituents connected to the bus and resulting in system failure.
1.1.2 RS-232 Communication
The RS-232 communication protocol is constantly on the withstand the exam of your time. Extensive utilisation of the protocol for PLCs in industrial environments have started demanding RS-232 transceivers with smaller form factor, faster speed, lower power, cost efficiency, robustness, reliability, and ease of use. Faster and lower power include reducing RS-232 voltage levels from ±12 V to ±5.4 V
Majority of RS-232 transceivers require compatibility with power supplies down to 3.3 V and employ charge pumps to boost and invert that voltage to RS-232 voltage levels that are suitable for the conventional > ±5 V.
The trend in recent years would be to reduce power dissipation, especially in portable applications. Therefore, board designers have been reducing power-supply voltages or eliminating them altogether whenever we can. Lowering supply-voltage levels is a trait common to the semiconductor industry. Consequently, the burden of generating the necessary RS-232-compliant voltage swings is around the RS-232 driver IC. This can be done by having an internal or external charge pump or DC-DC converter. TI devices incorporate the RS-232 driver and charge pump in a single solution which makes it simple to design a circuit board only using a single supply.
1.2 System Description
The TIDA-01230 reference design combines both data and power isolation together with RS-232 communication interface to provide a robust, low-cost, low footprint solution that customers can directly put on their designs. This TI Design combines a High-Performance Reinforced Digital Isolator With Integrated High-Efficiency, Low Emissions DC-DC Converter (ISOW784x) having a Robust 3.3-V TwoChannel RS-232 Transceiver ( TRS3122E ).
The TIDA-01230 does not require any extra components to create the isolated power. This will make the solution just one quarter of the size all existing solutions utilizing a discrete transformer to create the required isolated power. This TI Design requires a single power supply input (3 to 5.5 V) and digital signals referred to the input supply level. The board generates a remote power using an integrated DC-DC converter. This isolated supply is used to power the RS-232 transceiver. The input signals are isolated and attached to the transceiver, which converts digital signals into its RS-232 counterpart. Similarly, RS-232 signals fed at the transceiver end are transformed into corresponding digital signals.
PLCs and grid communication systems happen to be a fundamental element of factory automation and industrial process control. They will use digital and analog I/O modules to interface to sensors, actuators, and other equipment. Analog inputs for that PLC system include temperature sensors and transmitters, current sensors, voltage sensors, yet others that can convert an actual quantity to an electrical signal. Digital inputs include push-buttons, proximity switches, photo sensors, pressure switches, and more. These signals have to be either group isolated or per channel isolated. PLCs are expected to operate in harsh industrial environments. Hence sensor signals are transformed into digital domain and coupled through a digital isolator to manage domain. For sensor signal conditioning, power is generated from the backplane side by an isolated DC-DC converter. If multiple channels are present and all sorts of are isolated from each other, then use multiple of these isolated power solutions, which increase solution size and cost and produce down reliability because of increased component count. Gelling the TIDA-01230 can significantly lessen the required area and produce down the BOM cost through the elimination of the requirement for discrete transformers.
Medical equipment need isolation patient safety. For ECG, multiple leads are attached to the patient. The signal chain should be robust enough to capture very weak signals, digitize and process it, and pass it somewhere controller over the isolation barrier. The power for signal processing units is typically generated by aboard isolated DC-DC converters. This power can be replaced directly using the TIDA-01230 to obtain both communication and power isolation with the RS-232 interface. Also any peripheral connected to this equipment must be isolated from the controller. This solution makes sense to digitally isolate the interface signals and also power up the transceivers.
1.3 Key System Specifications
Table 1. Key System Specifications
| FEATURE | SPECIFICATION | VALUE |
| Input supply | Input voltage | 3 to 5 V |
| Isolated output supply | Output voltage | 3.3 V |
| Output current | 75 mA (3.3 VIN) | |
| 130 mA (5 VIN) | ||
| Communication | Max data rate | 1 Mbps (four channels) |
| Isolation | Clearance | > 3 mm |
| Working voltage | 300 VRMS | |
| CMTI | > 100 kV/us | |
| EMI or EMC | ESD | 6 kV (across barrier) |
| 8 kV (on RS-232 lines) |
1.4 Block Diagram
Figure 1 shows the high-level block diagram from the TIDA-01230. As earlier mentioned, this TI Design could be broken down into two main sections: the isolation block (ISOW7842) and the interface block ( TRS3122E ).
The TIDA-01230 creates a single input of 3 to 5.5 V. The integrated power supply can produce an isolated output voltage of 3.3 V (for any input) or 5 V (for VCC1 ≥ 5 V). This can be used to power up the RS-232 interface block. The TIDA-01230 communicates with digital signals on one side and gives the RS-232 lines alternatively. The design could be interfaced to an MCU transmitting and receiving data using both channels (two for transmitting and 2 for receiving data). These signals are isolated in the RS232 lines readily available for communication towards the field side. The side A of the isolator is powered from the 5-V or 3.3-V power supply, the secondary side from the isolator will be powered through the integrated isolated power supply. The integrated power supply can establish two voltage configurations on side B: 5 V or 3.3 V for that 5-V supply on side A, and 3.3 V for that 3.3-V supply on side A.
The load in this TI Design may be the RS-232 transceiver. The look can interface with the MCU to transmit and receive data and enable transmission and reception. The signals back and forth from the MCU are isolated from the transceiver. The TIDA-01230 board could be divided to two main sections: the isolation block and the interface block.
1.5 Highlighted Products
The TIDA-01230 features the next devices from Texas Instruments.
1.5.1 ISOW7842
The ISOW784x is really a group of high-performance quad-channel reinforced digital isolators by having an integrated high-efficiency power convertor. The integrated DC-DC convertor provides as much as 650 mW of isolated power at high efficiency and could be configured for a number of input and output voltage configurations. These units eliminate the requirement for another isolated power in space-constrained isolated designs. ISOW784x devices provide high electromagnetic immunity and low emissions, while isolating CMOS or LVCMOS digital I/Os. The signal isolation channel includes a logic input and output buffer separated with a silicon dioxide (SiO2) insulation barrier, whereas power isolation uses on-chip transformers separated with a thin film polymer as insulation material. These units prevent noise currents on a data bus or other circuits from entering the neighborhood ground and disturbing or damaging sensitive circuitry. Through innovative chip design and layout techniques, the electromagnetic compatibility from the ISOW784x continues to be significantly enhanced to help ease system level ESD, EFT, surge, and emissions compliance. The high efficiency of the power convertor allows operation in a higher ambient temperature. The ISOW784x group of devices will come in a 16-pin SOIC wide-body (DWE) package.
o Integrated high-efficiency DC-DC convertor with on-chip transformer
- Wide input supply range: 3 to 5.5 V
- Regulated 5- or 3.3-V output
- Up to 0.65-W output power
- 130-mA load current (5 VIN to five VISO; 5 VIN to 3.3 VISO
- 75-mA load current (3.3 VIN to three.3 VISO)
- Soft start to limit inrush current
- Overload and short-circuit protection
- Thermal shutdown
- Signaling rate as much as 100 Mbps
- Low prop-delay: 13 ns typical (5-V supply)
- High CMTI: ±100 kV/us min)
- Robust electromagnetic compatibility ( EMC )
- System level ESD, EFT, and surge immunity
- Low radiated emissions
- Safety-related certifications
- 7071-VPK Reinforced Isolation per DIN V VDE V 0884-10 (VDE V 0884-10): 2006-12
- 5000-VRMS Isolation for One minute per UL 1577
- CSA Component Acceptance Notice 5A, IEC 60950-1 and IEC 60601-1 End Equipment Standards
- CQC Approval per GB4943.1-2011
- TUV Certification based on EN 60950-1 and EN 61010-1
- All agency certifications are planned
- Extended temperature range: -40°C to 125°C
1.5.2 TRS3122E
The TRS3122E is really a two-driver and two-receiver RS-232 interface device, with split supply pins for mixed voltage operation. All RS-232 inputs and outputs are safe to ±15 kV while using IEC 61000-4-2 Air-Gap Discharge method, ±8 kV while using IEC 61000-4-2 contact discharge method, and ±15 kV while using human-body model. The charge pump requires five small 0.1-μF capacitors for operation from as low as single.8-V supply. The TRS3122E is capable of doing running at data rates up to 1000 kbps, while maintaining RS232- compatible output levels.
- Extended VCC operating nodes: 1.8 V, 3.3 V, or 5.0 V
- Unique tripler charge pump architecture enables low VCC of just one.8 V while maintaining compatibility with 3.3-V and 5-V supplies.
- Integrated level-shifting functionality eliminates the need for external power or additional level shifter while interfacing with low-voltage MCUs
- Enhanced ESD protection on RIN inputs and DOUT outputs
- ±15-kV IEC 61000-4-2 air-gap discharge
- ±8-kV IEC 61000-4-2 contact discharge
- ±15-kV human-body model
- Specified 1000-kbps data rate
- Auto powerdown plus feature
- Low 0.5-uA shutdown supply current
Meets or exceeds compatibility requirements of RS-232 interface
2 Getting Started Hardware
2.1 Board Description
Figure 2 shows how big the isolated RS-232 by having an integrated power solution. The solution space inside the board continues to be marked in red and is slightly larger than the ISOW7842 IC. The board including all the connectors measures 21.1 × 12.7 mm while the actual solution space (marked in red) is 14.8 × 11.5 mm.
2.2 Connectors Description
Table 2. Connectors
| CONNECTOR | PIN POSITION | PIN NAME | I/O OR
POWER |
DESCRIPTION |
| J1 | 1 | VIN | Power | Input supply (3 to 5.5 V) |
| 2 | Tx1 | I | Digital driver input | |
| 3 | Tx2 | I | Digital driver input | |
| 4 | Rx1 | O | Digital receiver output | |
| 5 | Rx2 | O | Digital receiver output | |
| 6 | GND1 | Ground | Input supply ground | |
| J2 | 1 | VISO | Power | Isolated power output |
| 2 | TxD1 | O | RS-232 output | |
| 3 | TxD2 | O | RS-232 output | |
| 4 | RxD1 | I | RS-232 receiver input | |
| 5 | RxD2 | I | RS-232 receiver input | |
| 6 | GND1 | Ground | Isolated output ground |
The design comes pre-populated with two TI ICs (ISOW7842 and TRS3122E). VIN and GND1 are connected at Pin 1 and Pin 6 of J1, respectively. Two channels are available for transmitting using RS-232 and two channels for receiving RS-232 signals. The digital inputs does apply to Pin 2 and Pin 3 of J1 and the digital counterparts of the received RS-232 signals could be observed on Pin 4 and Pin 5 of J1. On the other hand of the board, the corresponding RS-232 signals for that digital inputs from the MCU side could be observed at Pin 2 and Pin 3 of J2. Similarly, the RS-232 input signals could be given at Pin 4 and Pin 5 of J2. The boards include male berg heads to permit bus cables to become securely attached.
Connect MCU Tx lines to Pin 2 and Pin 3 of J1 and MCU Rx lines to Pin 4 and Pin 5 of J1. The related RS-232 line is available as four separate connections at J2, which may be connected directly to end system using required cables. The TI Design has four channels that can be used depending on the connections in the MCU side.
This TI Design could be extended with other RS-232 transceivers by replacing the present transceiver around the board deciding on the best voltage configuration. The isolated output voltage configuration can be easily changed by changing the resistor combination of R1 and R2. See Table 3 to understand how you can realize the necessary voltage configuration.
Table 3. Voltage Configurations
| INPUT VOLTAGE (V) | OUTPUT VOLTAGE (V) | R1 (Ω) | R2 (Ω) |
| 5.0 | 5.0 | Populate | Do Not Populate |
| 3.3 | Do Not Populate | Populate | |
| 3.3 | 3.3 | Do Not Populate | Populate |
| 5.0 | Invalid configuration | Invalid configuration |
3 Testing and Results
3.1 Test Setup
3.2 Communication Functionality
Waveforms were captured at max data rate from digital to RS-232 side.
3.3 Propagation Delay
3.4 IEC-ESD Performance (IEC-61000-4-2)
Contact discharge IEC-ESD testing was performed on the TIDA-01230 utilizing a pointed contact discharge tip. The board was put through mainly 2 kinds of ESD strikes. In the “Across barrier” row, the interface side was struck using the reference ground or power earth connected in the MCU side ground ( GND 1). Similarly for the “Same side” row, the interface lines were struck with the reference ground or power earth attached to the interface side ground (GND2). Table 4 shows the IEC-ESD test results. Each of the stressed pins were put through 10 strikes of every polarity from the corresponding stress voltage.
Table 4. IEC-ESD Results
| NATURE OF STRIKE | STRIKE POINT | STRESS VOLTAGE | RESULT |
| Across barrier | VISO | ±8 kV | Pass |
| TxD1 | ±8 kV | Pass | |
| TxD2 | ±8 kV | Pass | |
| RxD1 | ±8 kV | Pass | |
| RxD2 | ±8 kV | Pass | |
| GND2 | ±8 kV | Pass | |
| Same side | TxD1 | ±8 kV | Pass |
| TxD2 | ±8 kV | Pass | |
| RxD1 | ±8 kV | Pass | |
| RxD2 | ±8 kV | Pass |
3.5 Radiated Emissions (CISPR 22)
The emission measurements were done based on CISPR 22 standards inside a certified facility.Figure 8 shows the emissions measurement for that TIDA-01230 once the communication lines were being operated within static high condition.
Article Courtesy: Texas Instruments