The MultiProtocol Logic Probe provides multiple analyzer functions in a compact instrument that can be held like a large pen. Only about 1.5" wide and 6" long, it can quickly probe and save hundreds of waveforms in its 512 KB memory. Using an FPGA in conjunction with the Z80S183 serial ports and analog inputs, the Probe can save digital or analog data and display it in logic analyzer form, serial protocol analyzer form (e.g., for RS-232), digital voltmeter, digital counter, or oscilloscope form (up to the 500 Khz sampling rate of the Z80S183). A unique FPGA digital sampling circuit permits 200 MHz logic level data acquisition from the probe tip. Additional connector pins allow an external clock, trigger, and 16 bits of sample probes to be attached to the MultiProtocol Logic Probe as needed. An optional serial computer interface is also provided.

Purpose of the MultiProtocol Logic Probe

What happened to the old logic probes of long ago? Turning on and debugging printed circuit boards used to be fast and easy with schematics, a logic probe, and a logic analyzer for the really tough problems. When my old logic probe broke after decades of service, I tried to buy a replacement—but no manufacturer makes these any more! Yes, you can now buy portable logic analyzers and scopes, but nothing beats the simple handheld logic probe for quick checking of multiple circuit points for signal levels and activity. So, I decided to build one—and to add to it the most common logic analyzer, oscilloscope, voltmeter, frequency counter, and protocol analyzer functions into a powerful instrument that can be held like a pen. The result is the MultiProtocol Logic Probe, which uses a synergistic combination of the Z80S183 analog and serial capabilities with the high-speed sampling of the Altera APEX FPGA device.

The block diagram of the MultiProtocol Logic Probe is shown in Figure 1. The probe tip goes to the FPGA, an Altera 20K100X with a PLL running at 200 MHz, as well as to one of the serial ports of the Z80S183, and to a buffering op-amp that then goes to one of the analog input of the Z80S183. The FPGA has the probe tip sampling circuit designed to use a sophisticated algorithm to capture data reliably at 200 MHz. In addition, the probe tip signal goes to a 32-bit counter/timer circuit in the FPGA that tracks how many pulses have occurred, how wide (in 100 MHz clocks) the pulses are, and a moving average frequency of pulses. Of course, the display also indicates 0/1/Z in simple logic probe mode. The FPGA also includes circuitry to load the selected data format into a 4-Mbit high-speed SRAM. An EPROM for the Z80S183 program code and configuring the FPGA also are provided. The second serial port is used to send data to an LCD display module that shows the selected results depending on the current operating mode. Connectors are provided for external trigger, clock, and 16 signal logic analyzer probes, as well as a computer serial interface (using the CSIO capability of the Z80S183).

To keep operation of the probe easy to learn and fast, I provide a very simple three-button interface. One button cycles through the operating modes, a second button selects a function within that mode, and a third button is the execute button. There are currently 6 operational modes:

With this kind of power in this tiny instrument, no one needs to lug around and hook up big boxy instruments at the test site. Small and inexpensive enough that every engineer can have one, the upgradeable MultiProtocol Logic Probe has all the capability needed to debug most common problems on today’s projects.