Design Engineering

Frédéric Vecoven realised a need for a replacement display whilst developing a new OS for the Roland Super JX instruments and came up with a design concept. Guy Wilkinson took over the design and completed it at the end of 2015.

Software

Whilst Fred developed the replacement Super JX operating systems, he came up with a sophisticated software toolset that runs under OSX or Linux. It included graphical display simulation as well as MIDI and could also pipe data to real hardware for exhaustive testing. The Vecoven OS is an astonishing piece of engineering that squeezes every last drop of resource out of an ancient micro-controller and very limited memory.

The software toolset and emulated Super JX code is used extensively for display development and is run alongside real hardware for JX10 or MKS70.

FunWork

The software contains a boot-loader that error checks the display application at power up. When using a Vecoven OS, updates can be made via sysex data using the boot-loader.

Embedded code is broken down into a large number of small modules and although is written in C, a class based coding style has been applied for easy navigation and scalability for forthcoming projects. A set of data tables guide the display layout arrangements with code interpreting the settings and rendering the display. Once the underlying code was written, each screen was configured very easily to produce the whole set of displays and menus required. As usual for a development project, “display just a bit of text” turned into a sizeable task once the creativity and perfectionism started creeping in. For examples; Four different justified fonts, pixel accurate centralised text, shadow boxes and lines.  Just for fun, MISRA coding rules were applied as well as cross platform techniques, to run on other processors such as emulating it on the PC. We went to town on such a straightforward task but the results are well worth the extra time spent and hopefully bug free too.

Linux Mint was the platform of choice for display code development. Code development testing used Microchip MplabX Embedded Development Environment in C and assembler. The GCC compiler provided an environment for emulating display and Super JX code when not running on target hardware.

A YouTube video showing the development of the display software before a PCB was worked out is shown below. The electronics was laid out on breadboard using a PIC2520 microcontroller. If you look closely at the old and new videos, they are slightly different because once the design had been installed in a JX10, the “ease of use” factors started to become important and new layouts were born.

 

Mechanical

The PCB has been designed to fit in place of the old display tube and sit flush against the Roland Display Board. This way it can act as a method of lining up the display very precisely within the window because pins extending from the underside of the PCB fit into the old VFD tube connection holes.

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It eases the burden on the installer, making for a very easy fitment without any measurements or mechanical trials needing to take place. Furthermore, wiring is minmised and the Display PCB doesn’t have to be located somewhere else in the instrument.

The PCB can also be used on its own with a 14 way IDC flat cable and has M3 mounting holes to allow for this arrangement.

Electronic

Electronically the display is very simple but still has a number of very important details to ensure it’s reliability when installed inside the existing design.

The diagram below shows the internals of the Display PCB and consists of 3 main areas:

Electronics

Microcontroller

This takes commands and data from the assigner or Roland display board and interprets them before rendering the images on the display. A parallel interface, optimized for speed connects to the display.

The microcontroller contains font and symbol data as well as many formatting arrays that guide positioning of items on the display. Should the Vecoven OS menu system be developed further, additional layouts and menus may be added easily and downloaded by users.

The software also caters for lower cost or surplus character displays, details are shown in a later section.

Isolator

This is necessary to isolate the noisy VFD circuitry from the instrument. If this is not present, then additional noise can be heard on the audio path. It also allows the display module to be powered from a completely different source if users require to reduce burden on the instrument power circuit.

Power Supply

A modern graphic VFD consumes huge gulps of power during operation and power on. A DC-DC power circuit provides the necessary high current on demand whilst working from a wide input voltage range without causing display instability issues. It was noticed early in the project that the power source for the VFD has to be robust and deliver a very precise voltage under low and high current conditions.

A further improvement was a soft start feature that prevents high current consumption at power on to prevent stressing of the transformer fuses and main input rectifier. This is due to the VFD heating filaments pulling high current as they warm up at power on.

Using this carefully designed power arrangement allows the display to be powered reliably by the instrument power supply or an external source without impacting the noise floor or stressing the existing instrument power circuits.

Design Spark PCB 7.0 was used to capture the schematic and design the PCB layout. The Power supply was modeled in Texas Instruments Webench and some adjustments to component types were made after using a thermal imaging camera to examine temperature rise and a Siglent 100MHz Oscilloscope to examine switching characteristics.

 

Read More: Testing

 

Copyright © 2016 Super Synth Projects, Guy Wilkinson

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