Radio Transmitter

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Tuesday, May 5, 2009

GPIB to RS-232 converter

This project fills the need of anybody who has a test instrument with the GPIB port and likes to get the screen dump on his PC without any GPIB card. It emulates the HP7470A operation on the GPIB side, and outputs the HP-GL data at the RS-232 port to be read and stored on the PC by any suitable software. The operation of this interface is not just limited to plotter emulation: any data intended to be received by a GPIB Device (addressable or listener only) can be captured and brought out to the RS-232 port, including raw data from the instrument or rasterized data for a GPIB graphic printer. GPIB addresses and other set-and-forget parameters can be modified and permanently stored using a simple setup menu. It is based on a PIC16F628A microcontroller, and the PCB size is just 7x7.5cm.


The hardware of Pic-Plot interface is quite simple: the active components are just a PIC16F628A, a 5V regulator and three transistors. External connections are a GPIB connector, a Serial port and a DC power connector.

The microcontroller does all the necessary jobs to emulate GPIB Device functionality, in both Listener and Talker mode, by recognizing addressing, commands and managing the Handshake lines. Controller mode is not needed for the intended functionality, and therefore is not supported. Once the device is addressed and it receives data from the Talker, the same data are forwarded to the COM port at 9600 baud: the hardware UART inside the PIC16F628A generates the serial data going to the PC through the RS232 port. Only in Setup mode the data flow is bidirectional at the same baud rate. The RS-232 connector on the interface is a standard male DB-9, and should be connected to the PC using a null-modem female-to-female serial cable.

PCs missing the COM port but equipped with an USB port can be still used with the aid of an inexpensive USB to serial converter, provided that the necessary Virtual Com Port drivers are properly installed.

A jumper is provided to enter Setup mode: when Pic-Plot it powered with this jumper in the closed (short) position, then it starts-up in Setup mode. In this mode the microcontroller UART is used to read/change a few set-and-forget parameters. GPIB cable can be left connected to the instrument, but in Setup mode the GPIB port is not monitored by the Pic-Plot. For normal operation this jumper must be left open. More details about Setup mode can be found in the USE AND OPERATION section.

Power supply can be any voltage between 8 and 16V, and current drain is far below 20mA. With such a large supply requirements, a low-cost unregulated 12V wall adaptor can be used as a power source. Connector polarity is center positive (+). An interesting possibility for those who use the USB-to-serial bridge is to bypass the Pic-Plot 5V onboard regulator and spill the 5V supply from the USB connector mounted on the bridge. This solution of course asks for a simple modification of the bridge or a modified USB cable, then it is suggested only to people having the necessary technical skills to do things right. You can find details by clicking here, or you might prefer to see our new Pic-plot2 which directly supports GPIB-USB conversion.

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