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Pocket-Sized Digital Oscilloscope Adapter

May 8, '98
Last update: May 31, '99


This is a portable oscilloscope adapter that it can be held in breast pocket. Its operation is only sampling and sending to host PC. The most of functions of the oscilloscope are processed by host PC. Therefore, oscilloscope adapter can be simplified.


Specifications
Acquisition8bit, 10ksa/sec (fixed)
Input Voltage Ranges1mV/lsb (256mVp-p/fs) Zi=1Mohm
3.3mV/lsb (853mVp-p/fs) Zi=1Mohm
10mV/lsb (2.56Vp-p/fs) Zi=10Mohm
30mV/lsb (8.53Vp-p/fs) Zi=10Mohm
100mV/lsb (25.6Vp-p/fs) Zi=10Mohm
300mV/lsb (85.3Vp-p/fs) Zi=10Mohm
Host InterfaceRS-232C (9pin)
Transmit: 115kbps, Receive: 9600bps
Between Internal circuit and Interface circuit
are isolated with optical-isolaters.
Storage and DisplayHost PC takes charge of all of them.
Power RequirementBattery: a 9V alkaline battery
Operating current: 15mA(10:1), 25mA(1:1)
No power switch. It is controlled by host PC via RS-232C.
Cost3,000 Yens (parts)

Key Techniques

I saied that this is a simple A/D converter at top of this document. This means software quality affects to convenience to use.

But I think it has every advantage that most of processes are taken charge of by host PC. Sampling rate of 10ksa/sec is also slow for software processing, so that latest processors will be able to processes in real time with ease.

The sampling rate of 10ksa/sec is dissatisfied as a digital oscilloscple. This restriction on sampling rate is by be linked between acquisition unit and host PC with the RS-232C. However, it will be useful on the lower frequency signal.

Negative and positive powers for the analog circuit is generated with charge pump of the MAX232AC. The MAX232AC is useful when using OPAMP with single +5V power supply.

For range changing, this is controlled by host PC with atteneter and amplification value control commands. For others there is no especialy technique, only amplitude input signal, sampling with microcontroller with integrated ADC, and sending wave form data to host.

Control Program

This image of the wave form is displayed with sampling control program. It is a current waveform that acquired at the switch board with a current sensor.

Peaks that stand out from sine wave are input current of the PC that it has switching power supply. Sine wave is currernt of fluorescent lamps, refregerator and others. They are inductive load so that phase of their current is lagged. The functions of the control program are as follows.


When build it

This oscilloscope adapter using a TMP47P242VN 4bit microcontroller for acquisition control. But it is difficult to obtain and to programm personaly, so that I recommend to replace the microcontroller with any other microcontroller, such as PIC and AVR, when you want to follow this project. The details of the acquisition controller is as follows.

Communication format between host PC and oscilloscope adapter is N81 (no parity, 8 data bits, one stop bit). Data transfer rate between oscilloscope adapter and host PC is 115.2kbps. This is the minimun required transfer rate for real-time transmission of sampled data of 10ksa/sec. And command transfer speed of from host PC to oscilloscope adapter is 9.6kbps.

The acquisition controller will enter state of command waiting after power-up. One byte of command value range is from 0x50 to 0x5F, any other value will be ignored. The controller outputs lower four bits to P01-P03, wait for one milisecond, and enter to sampling state (sampling loop). In sampling state, anything data is raceived from host PC, acquisition controller will return to command waiting state. Host PC must send a 0x00 before sending any command.

When above protocol is satisfied, control program will be able to use without any patches. But remaking the controll program as windows based application will better than DOS based application.

Technical Data

Circuit Diagram
Control program (includes source codes)

Sign