Gpibgrok

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. There is only one GPIB-USBTMC interface that we know of: the TEK-USB-444 from Tektronix, and it's ridiculously overpriced at around $740.

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

• Using an ARM Cortex-M3 microcontroller would get us:
  • Built-in USB
  • Plenty of horsepower to handle the throughput a GPIB device will reasonably need
  • Many different implementations to choose from, and many inexpensive development boards
  • Can start with an existing development board + GPIB connector
• Voltage levels on GPIB pins is "negative logic with standard TTL levels": true <= 0.8V, false >= 2.0V. (to be verified)

Software

Due to the long history of the IEEE-488 and SCPI standards, there are many devices out there supporting some earlier version of the protocol, and these will typically support commands that are vendor-specific, and syntax that is not compliant IEEE-488. Therefore supporting various device-specific or vendor-specific "quirks" will likely be a big part of real-world use-cases.

Components

GPIB connectors

• Right-angle PCB mounted male connector at Digikey

GPIB protocol chips

• Philips/NXP HEF4738
• Fairchild 96LS488
• NEC uPD7210C/D
• National NAT9914

(pretty much all of them are no longer available)

GPIB transceiver chips

• National DS75160A
• SN75160B: Octal General-Purpose Interface Bus Transceiver (datasheet). Mouser: €1.55
• SN75161B: Octal General-Purpose Interface Bus Transceiver (datasheet). Mouser: €1.45
• SN75162B: Octal General-Purpose Interface Bus Transceiver (datasheet). Mouser: €2.03

Resources

• Massimiliano Gentile's thesis on writing a USBTMC driver for the AVR32 architecture.
• Hack a Day: GPIB connectivity twofer (covers Galvant GPIBUSB and Sven Pauli's RS232-GPIB interface)
• GPIB connector pinout