gpibgrok

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This page documents some ideas and information for a GPIB-USBTMC hardware interface.

Motivation

There are many ways to communicate with devices that have a GPIB port, and sigrok aims to support as many of them as possible (see IEEE-488). However in this day and age the only reasonable interface for this would have to use a USB device port, since USB host ports are so ubiquitous. The USB standards include a device class specifically made for test and measurement, called the USBTMC class.

Yet most of the GPIB-USB interfaces available don't use this device class; they typically use either a proprietary protocol or serial emulation. The originator on this project, Uwe_Hermann, thought that the TEK-USB-488 interface from Tektronix was being able to act as a controller (and it's ridiculously overpriced at around $740). But this interface is actually an USB host controller for a device already implementing USBTMC device class, and making it (and only this one) reachable through GPIB; e.g. making an USB-only oscilloscope implementing the USBTMC class reachable through GPIB. There is even an additional USB protocol to be implemented by this device to be able to control the GPIB address it will appear on. Thus, making a GPIB controller interfaced through this USBTMC class may not be possible, IMHO (Benoar).

We think we can make a GPIB-USBTMC interface that is:

  • 100% free and open source, hardware and firmware/software
  • 100% standards-compliant
  • Considerably cheaper than anything else out there (less than $50)

In addition, since we'd be making essentially a "server-side" i.e. USB device-side implementation of the USBTMC protocol, this code would be reusable in projects such as Das Oszi.

Hardware design

  • STM32L0 ARM Cortex M0
    • Built-in FS USB with crystal-less operation
    • Compatible with libopencm3
    • 32 pin LQFP keeps size low and assembly easy
  • Specialty transceivers
    • TI still makes the SN75160 (data) and SN75161 (handshake)
    • High current drive required for logic lines according to GPIB standard
    • Directional bundling of pins based on protocol simplifies firmware
  • Two-layer PCB to keep cost low
  • Designed to fit in L-COM housing for CIB24S GPIB connector
    • Saves effort on custom enclosure
  • Current design hosted at Upverter

Firmware

This is more of a TODO list at the moment:

  • Start with libopencm3
    • Add new class for USBTMC
  • GPIB
    • Implement functions
      • Serial poll
      • Write
      • Read
      • Reset
      • Local lockout
    • Monitor and handle SRQs
  • Commands
    • All data is just passed thru from PC to GPIB device and back, parsing of SCPI or other protocols is not required in firmware
  • Firmware update
    • Use the DFU bootloader in libopencm3
    • Modify to check register for boot flag instead of hardware pin
    • Some special command in USBTMC can set to bootloader mode and reset processor, then when DFU is done, set flag back and boot to main

Components

GPIB connectors

GPIB protocol chips

(Pretty much all of them are no longer available, or horribly expensive, or hard to get, or a combination thereof.)

GPIB transceiver chips

Bill of materials

Description Manufacturer Part Number Package Quantity Vendor Price
Generic Capacitor 100nF 10V any 0603 5 $0.10
Generic Capacitor 1 µF 10V any 0603 2 $0.10
Chip LED Lite-On LTST-C193TBKT-5A 0603 1 Digi-Key $0.47
SMT Micro USB Female right-angle bottom-mount connector type B Molex 473460001 Receptacle 1 Newark $0.52
Connector, GPIB Sldr, Female L-COM CIB24S 1 L-COM $8.02
Generic Resistor (0.0R) any 0603 2 $0.10
Generic Resistor (30) any 0603 1 $0.10
Common mode filter with ESD protection for USB 2.0 interface STMicroelectronics ECMF02-4CMX8 QFN-8L 1 Arrow $0.54
Octal Transceiver GPIB Control Texas Instruments SN75161BDW SOIC 1 Arrow $1.60
LP2980-N Micropower 50-mA Ultra Low-Dropout Regulator in SOT-23 Package Texas Instruments LP2980IM5-3.3/NOPB SOT-23-5 1 Arrow $0.43
Octal Transceiver GPIB Data bus Texas Instruments SN75160BDW SOIC 1 Digikey $2.18
32-bit MCU ARM Cortex-M0+, 64 KB Flash, 8 KB SRAM, 2 KB EEPROM, USB STMicroelectronics STM32L062K8T6 LQFP32 1 Arrow $2.33

Resources

Volunteers

Please edit this section with your name and (public) contact details if you are interested in developing this.

  • User:Ivan747 (same nick on IRC), experience in hardware, low level firmware (but not USB). Willing to participate with the support from someone experienced in USB protocols on any kind of ARM device. Contact: ivan747[at]users.sourceforge.net.
  • User:Elbridge (same nick in IRC), experienced with hardware and firmware, but new to open source.
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