Simple and cheap OPC-478 interface roomed into a DB9 by Maurizio Malaspina (IW6DFW), April 2008

Transcription

1 Simple and cheap OPC-478 interface roomed into a DB9 by Maurizio Malaspina (IW6DFW), April 2008 Building this simple circuit you will be able to link your Icom radio to PC to either remotely control your rig or simply to manage its memory. Referring to somebody who doesn t know what the OPC478 interface is, I clarify that it permits to exchange data between predisposed Icom radios and a PC equiped with a proper software able to manage the radio, even from a remote stationing using a virtual instrument, or simply to save, edit and update its memories. In this short paper, I m not talking about a list of existent application softwares finalized to aid the board introduced. In this context I m limiting to mention the existence of original tools by Icom, that could be purchased with the rig, and about the capability of use free softwares, which is easily retrievable in the net [1]. The circuit Image 1 showes the circuit realized, inspired by a David Aldridge G3VGR s [2] work, who realized a variant of Jiry Holy OK2WY s original design. The main features of this schematic is given by the auto feed ability directly from the serial port of the PC (note that both DTR and RTS signals have to be properly handled by the software) and the opportunity to manage the Push To Talk (PTT) signal of the rig. The original project has been enriched by designing a particular PCB routed by Mr Franco Carisdeo, mounting SMD components instead of through-hole ones. The final result has been successfully tested on an Icom IC-E90 handheld device connected to a PC running IC-90 Memory Management Utility software r developed and sold by Matteo Campanella, IZ2EEQ [3]. Some laptops couldn t be able to properly supply this interface to make it works well, because of serial port voltage levels could be out of RS232 +/- 12V standard; despite of I want to specify that the homebrewed prototype has successfully passed a lot of tests performed on different kind of laptops. Image 1: OPC-478 interface schematic All the signals in the schematic above are referred to the common ground GND (serial port pin 5). There are two main sections: the serial data asynchronous transceiver and a PTT handler. In the first one, D1 diode acts as DTR power supply polarity inversion protection (as well-known the envelope of the UART signals could be both positive and negative respect to the common reference). Capacitor C1 stores a sufficient charge to assure power supply stability. When the Tx pin of the

2 serial port is driven high by PC (logic 0), D2 diode isn t biased and base-emitter junction of Q1 is forward polarized by R3 so that Q1 works into saturation area transmitting a low voltage value (almost 0V as V cesat ). In a dual way the Q2 feedback gives the currently transmitted voltage level newly inverted back to Rx pin, so that the PC can understand it in the original positive logic. With a dual reasoning, the transmission and the subsequent reception of a low voltage value (logic 1) works at the same manner; this time D2 protects the base of Q1 avoiding that the module of the base voltage becomes lower than its threshold voltage with reference to common ground. This schematic isn t fully UART standard compliant, despite of this, the PC can properly read the voltage levels received that are every positive and dependent of DTR signal voltage level provided by the PC. The second stage, used to manage the PTT of the rig, it s a simple open-collector based on Q3, that has its base terminal protected by both a clamp diode D4 and a D3 one acting as a snubber. Setting RTS signal high with the radio connected, a conductive path between PTT signal and common ground is built, switching the rig in tx mode; otherwise PTT signal should remain floating keeping the device in rx mode. Image 2 shows the waveforms detected by the scope (Yokogawa, model DL9040) during a transmission of a byte AA (hexadecimal value) at 9600bps without the radio connected. Yellow probe is the tx signal fetched on the TIP pin and green is the loopbacked rx signal detected on DB9 connector 2 pin. Image 2: Waveforms The signals above are, as waited, in phase opposition and their amplitude are almost 3V despite of DTR signal level is 5.7V in this measure. This behavior has a simple explanation because, for example, when the interface is transmitting a 0V level, there is a voltage drop across D1, moreover the collector of Q1 is float, so a conductive path is formed through R4, R2 and Q2 base-emitter junction. The propagation delay between the two signals, presents its worst-case during the rising edge of the feedback signal, reaching a value of 10µs. Who would like to try a fine-tuning of the two voltage levels, can attempt to change the values of both R4 and R1 resistors.

3 Realization of the prototype Image 3 showes the double-layers PCB mounting plane for SMD components. The prototype has been realized using a CNC milling machine (the related CAD file in gerber format are available in my home page [4] for download as a zipped archive). Image 3: PCB and serigraphy The routing of the tracks has been studied to directly solder a female DB9 connector using both the metal layers. The wired connections have to be implemented by soldering the cable directly in the pads (the four ones vertically aligned in the serigraphy above). All the used components are surface mounting device in low power version. If the board will be home-brewed, it s necessary keep in mind that the through holes must be realized using a metal conductor soldered in both top and bottom sides to assure proper electric contacts for the nets routed in both sides of the PCB. The suggested mounting sequence starts from resistors, diodes and transistors to end with electrolytic capacitor, the DB9 connector and the cables. Follow the bill of materials: BOM R1,R2,R4,R5,R6 4,7 KΩ SMD /4 W ±5% R3 10 KΩ SMD /4 W ±5% C1 47µF electrolytic SMD 5x5.5mm 16 V ±20% 85 C D1,D2,D3,D4 LL4148 SMD MiniMelf Q1,Q2,Q3 NPN BC817 SMD SOT-23 CN1 DB9 female connector (cable version)

4 The assembled circuit have to be roomed into a fully-plastic package for DB9 connectors. Note that the connector that go to the radio can t be standardized because of its strict dependence on the model of the radio the user want to connect to; it s therefore necessary to refer to the user manual of the rig used to understand both which type of connector have to be mounted and which are the pin of the radio to be connected. The photo of the realized prototype, top and bottom side, are respectively shown in images 4 and 5, also the complete version is depicted in image 6. Image 4: Mounted prototype TOP side Image 5: Mounted prototype BOTTOM side

5 Image 6: The final result for the Icom IC-E90 handheld Conclusions What to say? A realization that is worth one s while, at least for its reduced dimensions in the optics of use handheld devices. For those that aren t able to realized this SMD version but are very interested in this kind of interface, I signal another version built with traditional components downloadable from my home page [4] in the HW section. Bibliography and related links [1] Icom Pro Memory Edit : Hamlib: [2] [3] iz2eeq@arrl.net [4] download OPC-478 clone interface, Aprile 2006