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.



SCHEMATIC AND DETAILS

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.

Digital Keypad Combination Lock

This simple circuit is the electronic version of the combination lock. Using the special purpose LS7220 digital lock IC, the circuit allows a 4 digit combination of your choice to activate a relay for a set period of time. This relay can then be used to trigger a lock solenoid, enable a starter button, open a motorized door, or many other tasks that require a momentary signal.



Parts

• C1 ------- 1 1uF 25V Electrolytic Capacitor
• C2 ------- 1 220uF 25V Electrolytic Capacitor
• R1 ------- 1 2.2K 1/4W Resistor
• Q1 ------- 1 2N3904 NPN Transistor 2N2222 (Substitute)
• D1 ------- 1 1N4148 Rectifier Diode 1N4001-1N4007 (Substitute)
• K1 ------- 1 12V SPDT Relay Any appropriate relay with 12V coil
• U1 ------- 1 LS7220 Digital Lock IC
• S1-S12 ---- 12 SPST Momentary Pushbutton Keypad (see notes)
• HD1 ----- 1 12 Position Header

Notes

• To set the combination, wire the appropriate switches to U1 pins 3, 4, 5 and 6 using the header. For example if S1 was connected to pin 3, S2 to pin 4, S3 to pin 5 and S4 to pin 6, the combination would be 1,2,3,4. Now wire all other unused switches across the header to pin 2 of U1. In this way you can create any 4 digit combination you want. Pin 2 is the reset pin, so connecting all unused keys to it assures that the entire combination must be reentered if an incorrect key is pressed.
• When the appropriate combination is entered, the relay is activated for a period of time determined by C1. The 1uF capacitor specified in the parts list will result in an on-time of roughly 5 seconds. Increase the value of C1 to increase this time.
• An easy way to make a keypad is to buy 12 PC board mount pushbuttons and then etch a PC board so that the buttons are in 4 rows of 3, similar to a telephone keypad. Place this in a case and then use a label maker or transfer letters to add your numbers to the tops of the pushbuttons. You can also use a pre made keypad but keep in mind that you need a pad which provides an output for each key. Most pads available have the keys connected to provide a row and column signal when they are pressed.

Computer Controlled Frequency Counter/Logic Probe

This circuit is a stable frequency counter accurate to 5 significant digits. The range is 0 - 30MHz with an input sensitivity of greater then 100mV. The probe connects to the PC serial port. So by using the crystal oscillator already present on your PC serial card and software calibration, the Probes' external circuitry is kept to a minimum. Probe 9 can also be used as a logic probe/analyzer using included software (LPROBE92.EXE).


Parts

• R1,R2,R3,R4---- 4 100K 1/4W Resistor
• R5 ------------ 1 10M 1/4W Resistor
• R6, R7 ------- 2 3.3K 1/4W Resistor
• R8 ----------- 1 390 Ohm 1/4W Resistor
• R9 ----------- 1 1M 1/4W Resistor
• C1, C4 ------- 2 0.1uF Ceramic Disc Capacitor
• C2, C3 ------- 2 100uF 16V Electrolytic Capacitor
• D1 ----------- 1 1N4148 Signal Diode Any 200mA silicon signal diode
• D2, D3 ------- 2 3.3V Zener Diode
• D4 ----------- 1 6.2V Zener Diode
• U1 ----------- 1 74HC00 Quad Highspeed NAND Gate
• U2, U3, U4 ---- 3 4021 8 Stage Shift Register
• U5 ----------- 1 74HC393 Dual Highspeed 4 Bit Counter
• U6 ----------- 1 4040 12 Stage Binary Counter
• MISC --------- 1 PC Board, Wire, Suitable Probe, DB9/DB25 Connector

• The software to use this probe can be downloaded using the following link. Note that this software is compiled for Intel x86 platforms and runs under DOS, Win95, Win98 and WinMe. It does not run under any Windows version based on NT including Windows NT 3.51, WinNT4, Win2K, Win2K3, WinXP and Windows Vista. This is because NT based operating systems do not allow direct hardware access.

Probe Software, Zipped, 19K

• SETPROBE.EXE is the frequency counter calibration program. To give accurate readings the Probe must be calibrated to your PC. SETPROBE.EXE calculates the constant error correction factor for the particular PC serial card the probe is to be used on. The frequency counter corrects for this slight constant error in crystal frequency by using the correction factor contained in PROBE.DAT. To calculate this correction factor, a reliable oscillator of known frequency (eg 2MHz Crystal Oscillator) is required. When CALIBRAT.EXE is run, the Probe will sample the frequency and then ask for the true frequency value in HZ. The frequency entered must be to 1 Hz accuracy (no decimal points) or an error will occur (for example "200123" not "200123.34" or "2003.421 kHz"). The program then calculates constant error correction factor and stores it to PROBE.DAT. Calibration is only necessary once.
• LPROBE92.EXE is the logic analysis program . Logic states are displayed in real time. This program runs best under DOS (not a DOS window). The sampling speed is adjusted by using the left and right arrow keys.

The three triggering modes are:

• TRIG: Starts each scan (left-right of screen) on a negative going edge of logic signal.
• KEY TRIG: Waits for a key to be pressed before beginning each scan.
• FREE RUNNING: Not triggered.

• To Toggle between these use the UP / DOWN arrow keys. To quit from LPROBE press escape.
• FPROBE92.EXE is the frequency counter program. The measured frequency is displayed in Hz with commas indicating KHz and MHz. To quit from FPROBE press any key.
• Serial port pinouts are as follows.