One-Wire RS-232 Half Duplex (EE Tip #135)

Traditional RS-232 communication needs one transmit line (TXD or TX), one receive line (RXD or RX), and a Ground return line. The setup allows a full-duplex communication. However, many applications use only half-duplex transmissions, as protocols often rely on a transmit/acknowledge scheme. With a simple circuit like Figure 1, this is achieved using only two wires (including Ground). This circuit is designed to work with a “real” RS-232 interface (i.e., using positive voltage for logic 0s and negative voltage for logic 1s), but by reversing the diodes it also works on TTL-based serial interfaces often used in microcontroller designs (where 0 V = logic 0; 5 V = logic 1). The circuit needs no additional voltage supply, no external power, and no auxiliary voltages from other RS-232 pins (RTS/CTS or DTR/DSR).Grun1-Wire-RS232-HalfDup

Although not obvious at a first glance, the diodes and resistors form a logic AND gate equivalent to the one in Figure 2 with the output connected to both receiver inputs. The default (idle) output is logic 1 (negative voltage) so the gate’s output follows the level of the active transmitter. The idle transmitter also provides the negative auxiliary voltage –U in Figure 2. Because both receivers are connected to one line, this circuit generates a local echo of the transmitted characters into the sender’s receiver section. If this is not acceptable, a more complex circuit like the one shown in Figure 3 is needed (only one side shown). This circuit needs no additional voltage supply either. In this circuit the transmitter pulls its associated receiver to logic 1 (i.e., negative voltage) by a transistor (any standard NPN type) when actively sending a logic 0 (i.e., positive voltage) but keeps the receiver “open” for the other transmitter when idle (logic 1). Here a negative auxiliary voltage is necessary which is generated by D2 and C1. Due to the start bit of serial transmissions, the transmission line is at logic 1 for at least one bit period per character. The output impedance of most common RS-232 drivers is sufficient to keep the voltage at C1 at the necessary level.

Note: Some RS-232 converters have quite low input impedance; the values shown for the resistors should work in the majority of cases, but adjustments may be necessary. In case of extremely low input impedance, the receiving input of the sender may show large voltage variations between 1s and 0s. As long as the voltage is below –3 V at any time these variations may be ignore.— Andreas Grün, “One Wire RS-232 Half Duplex,” Elektor July/August 2009.

Flexible I/O Expansion for Rugged Applications

WynSystemsThe SBC35-CC405 series of multi-core embedded PCs includes on-board USB, gigabit Ethernet, and serial ports. These industrial computers are designed for rugged embedded applications requiring extended temperature operation and long-term availability.

The SBC35-CC405 series features the latest generation Intel Atom E3800 family of processors in an industry-standard 3.5” single-board computer (SBC) format COM Express carrier. A Type 6 COM Express module supporting a quad-, dual-, or single-core processor is used to integrate the computer. For networking and communications, the SBC35-CC405 includes two Intel I210 gigabit Ethernet controllers with IEEE 1588 timestamping and 10-/100-/1,000-Mbps multispeed operation. Four Type-A connectors support three USB 2.0 channels and one high-speed USB 3.0 channel. Two serial ports support RS-232/-422/-485 interface levels with clock options up to 20 Mbps in the RS-422/-485 mode and up to 1 Mbps in the RS-232 mode.

The SBC35-CC405 series also includes two MiniPCIe connectors and one IO60 connector to enable additional I/O expansion. Both MiniPCIe connectors support half-length and full-length cards with screw-down mounting for improved shock and vibration durability. One MiniPCIe connector also supports bootable mSATA solid-state disks while the other connector includes USB. The IO60 connector provides access to the I2C, SPI, PWM, and UART signals enabling a simple interface to sensors, data acquisition, and other low-speed I/O devices.

The SBC35-CC405 runs over a 10-to-50-VDC input power range and operates at temperatures from –40°C to 85°C. Enclosures, power supplies, and configuration services are also available.

Linux, Windows, and other x86 OSes can be booted from the CFast, mSATA, SATA, or USB interfaces, providing flexible data storage options. WinSystems provides drivers for Linux and Windows 7/8 as well as preconfigured embedded OSes.
The single-core SBC35-CC405 costs $499.

Winsystems, Inc.

Programmable Logic Controller Board

SmartTILE illustration 6.psdThe SmartTILE (Smart TRi Integrated Logic Engine) is a programmable logic controller CPU board that plugs onto a carrier I/O board. The board integrates a 32-bit CPU, ferroelectric RAM (FRAM) and flash memory, a battery-backed real-time clock, and an Ethernet port on board. Its digital, analog, and serial I/O signals are brought to a user’s carrier board via three sets of header pins.

All critical components are already built-in on board. A user just needs to design a simple carrier PCB that contains a D/A circuit that interfaces the SmartTILE’s low-voltage signals to real-world voltages and currents (e.g., 24, 120, or 240 V).

The SmartTILE-Fx provides 16 digital inputs, 16 digital outputs (5-V CMOS logic level), eight analog inputs, and four analog outputs (12-bit, 0-5V) and can be expanded to 128 digital inputs and 128 digital outputs. The controller board -Fx provides three channels of serial ports (3.3 V, TX, RX, and /RTS) that can interface to RS-232, RS-485, or even wireless radio. An I2C port (3.3 V) is also available, allowing OEM to interface to specialty ICs that support I2Cbus.

Contact Triangle Research International for pricing.

Triangle Research International, Inc.

RS-232 Serial Adapter for Android Devices

ACCESSThe ANDROID-232 is a USB serial interface board that enables you to control legacy RS-232 devices from your Android devices. The board is well suited for POS, gaming systems, retail, hospitality, automation, kiosks, defense industries, lighting, or any other application requiring the connection of RS-232 serial devices to an Android-compatible system.

The ANDROID-232 uses the Android Open Accessory protocol to “convince” an Android device that its on-board USB port (normally limited to USB slave or OTG modes) is actually an RS-232 port. This two-way data port enables external hardware to control the Android unit or the Android unit to control external hardware.

The ANDROID-232’s key features include an Android USB 2.0 full-speed host-to-industry-standard RS-232 DB9M serial port; support for a UART interface with RX, TX, RTS, and CTS; a 5,512-byte RX buffer size; a 256-byte TX buffer size; ±15-kV ESD protection on USB data lines and all RS-232 signals; status and fault LEDs including external power, charging status, and USB status; a Type-A USB connector; a latching 5-V external power input connector with an external regulated power supply; a –40°C-to-85°C standard industrial operating temperature; and RoHS compliance.

The board includes an Android sample program with source code. This program enables you to verify proper operation of the ANDROID-232 device, including sending and receiving RS-232 data. The ANDROID-232’s Python test program can cooperate with the Android sample program to verify proper receipt of transmitted data.

The ANDROID-232 costs $139.

ACCES I/O Products, Inc.

Industrial Temperature SBCs

EMACThe iPAC-9X25 embedded SBC is based on Atmel’s AT91SAM9X25 microprocessor. It is well suited for industrial temperature embedded data acquisition and control applications.
This web-enabled microcontroller can run an embedded server and display the current monitored or logged data. The web connection is available via two 10/100 Base-T Ethernet ports or 802.11 Wi-Fi networking. The iPAC-9X25’s connectors are brought out as headers on a board.

The SBC has a –40°C to 85°C industrial temperature range and utilizes 4 GB of eMMC flash, 16 MB of serial data flash (for boot), and 128 MB of DDR RAM. Its 3.77“ × 3.54“ footprint is the same as a standard PC/104 module.

The iPAC 9X25 features one RS-232 serial port with full handshake (RTS/CTS/DTR/DSR/RI), two RS-232 serial ports (TX and RX only), one RS-232/-422/-485 serial port with RTS/CTS handshake, two USB 2.0 host ports, and one USB device port. The board has seven channels of 12-bit audio/digital (0 to 3.3 V) and an internal real-time clock/calendar with battery backup. It also includes 21 GPIO (3.3-V) lines on header, eight high-drive open-collector dedicated digital output lines with configurable voltage tolerance, 16 GPIO (3.3 V) on header, two PWM I/O lines, five synchronous serial I/O lines (I2S), five SPI lines (two SPI CS), I2C bus, CAN bus, a microSD socket, external Reset button capabilities, and power and status LEDs.
The iPac-9X25 costs $198.

EMAC, Inc.