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 Updated  27Apr.2008

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GPIB to USB converter with the Pic-plot

Pic-plot based GPIB to USB converter

BACKGROUND

USB TO SERIAL CONVERTERS

THE USB CONNECTOR

POSSIBLE MODIFICATIONS OF THE ADAPTOR

SERIAL PORT CONNECTIONS

POWER TO THE PIC-PLOT

READY TO USE

Even if we have recently introduced the Pic-plot2 that directly converts GPIB to USB, who already has a serial Pic-plot might be interested to these notes.  You will be able to modify a commercial USB to serial dongle, so that it will convert to USB the RS232 data and simultaneously deliver the necessary power to the Pic-plot.

BACKGROUND

For those who own a PC or laptop without any Serial Com Port, it is still possible to use the Pic-plot by mean of an inexpensive USB to serial converter. These adaptors come in different shapes and cabling arrangements, but all of them offer the possibility, with a simple modification, to extract the 5V that is available from the USB connector. This allows to supply the Pic-plot, so that you can get rid of the AC-DC adaptor that otherwise would be necessary. You may also decide to put the modified adaptor and the Pic-plot together in a small box, then building by yourself a self-powered GPIB to USB converter.

USB TO SERIAL CONVERTERS

Of course there are so many different versions on the market that we cannot describe every one. And we do not need to choose a particular one, because what they all have in common is what we need: an RS232 and an USB connector. What can be different is the work we should do to pick the 5V line to be delivered to the Pic-plot. We report here three of the possible "flavors".

Type 1

USB "B" jack

RS232 male

Type 2

Pigtail with USB "A" plug

Pigtail with RS232 male

Type 3

Pigtail with USB "A" plug

RS232 male

THE USB CONNECTOR

What we need to do is to "steal" the 5V power from the USB adaptor to supply the Pic-plot. Therefore it is important to know the USB connector types and pinout.

USB jack connector types

USB plug connector types

Pin

Name

Cable color

Description

1

Vcc

red

+5V DC

2

D-

white

Data-

3

D+

green

Data+

4

GND

black

Ground

 

 

 

POSSIBLE MODIFICATIONS OF THE USB TO SERIAL ADAPTOR

Depending on the type of adaptor you have, and knowing where the 5V and the GND are available at the USB connector, you can easily make your own idea about the modifications you need. Some suggestion below are referred to the adaptors we have seen so far.

Type 1- using a cutter, it is possible to decap the soft plastic molded around the USB jack. A thin insulated wire can then be soldered on the inner terminal of Pin1.

Take 5V power from USB dongle

Type 2- USB side cable: Carefully split the outer insulation and the shield of the USB cable at the adaptor end. Locate the GND (should be black) and the +5V (should be red) wires. Do not cut them, but carefully remove a short length of insulation to allow soldering two wires for the Pic-plot supply. Use a multimeter to check you have really taken the wires that go to pin4 (GND) and 1 (Vcc) of the USB "A" plug. Restore insulations at the best you can.

Type 3- Same as type 2.

Bottom line, if you don't want to touch the USB dongle, you can just catch the the red and black wires inside an USB A-A extension or an A-B cable by carefully splitting the outer insulation.

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SERIAL PORT CONNECTIONS

To connect the USB adaptor to the Pic-plot you may either build a very short null modem cable or a pigtail (saving one female connector).

 

Pigtail wires will be soldered at the bottom side of the PCB to the indicated pins of DB9 Pic-plot connector. See the picture:

If you are using a Type2 adaptor, you may consider the possibility to cut the DB9 male of its RS232 cable and rewire as null-modem to a DB9 female that will be plugged in the Pic-plot serial port.

 

POWER TO THE PIC-PLOT

Pic-plot circuitry operates at 5V, but there is a 5V on-board regulator to allow the use of non-regulated wall-wart AC adaptors. To correctly supply it from the 5V taken from the USB dongle we must apply them at the IC1 regulator output. An handy point where to connect the 5V could be the jumper wire JP1 (see top overlay of the PCB). GND wire can be the same used for the serial data lines return.

 

READY TO USE

The assembled USB adaptor+Pic-plot is ready to use: the whole system is self-powered from the USB-host port, so you don't need the wall-wart. Don't forget to install the Virtual Com Port drivers on your PC following the procedures described in the USB adaptor instructions. Refer to our Pic-plot pages for its configuration.

You may want to put the complete system in a plastic enclosure: you can start from here:

 

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The projects presented in these pages are our own design and have been tested and verified by ourselves at the best we can. However, they might be inspired by concepts, ideas, solutions coming from known-art or free resources on the Web. We provide them as  reference designs to skilled hobbyists and technicians  who are willing to reproduce them for non-commercial use. Your results might be different from ours and we cannot be considered responsible for that. Similarly, we are not responsible for any damage or injury you might incur while building, assembling or using the equipments, projects or ideas presented in these pages. The firmware embedded in our projects is our property unless differently stated and, when available in the Download Area, it is license-free only for non-commercial purposes.  

Hotwater 2007-2009