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The Chroma CPU Plus (CC+): Alphanumeric Display

by Sandro Sfregola [21010294] <>
and David Clarke [21030085++] <>

Table of Contents


While a great synthesizer, the LED displays on the Chroma are sometimes cryptic and require that the user continually refer to either the Programming Manual or the Parameter Chart.

For instance, a user looking at the following data readout would not necessarily know (without additional assistance) that this shows a 'monophonic arpeggiate up keyboard algorithm.'

Another drawback to the existing displays is that only one parameter's data is shown at a time - thus making it difficult to appreciate the impact of other related parameters.

From the beginning, the CC+ was designed to be able to support an external alphanumeric display. The intent of such a display is to present parameter data in a much more user-friendly manner - helping to decode the significance of the settings, thus avoiding the need to continually refer back to source material.

By way of an example, the "P3    10" data above could be better displayed as:

Support for the Alphanumeric Display has been enabled and is available for use as of firmware version 214. The implementation decodes the data for the selected parameter – and dynamically updates the data as parameters on the Chroma are changed.


To take advantage of the alphanumeric display feature, 5 main items are required:

  1. A Chroma CPU Plus (CC+);
  2. Firmware version 214 or later;
  3. Installation of a second 16C550 UART chip;
  4. Installation of a display cable; and,
  5. Connection of the serial display unit.

All of these items can be undertaken by the end user and generally do not require much technical experience.

To obtain a CC+, see How Can I Get a CC+?

The firmware required is available on-line, free-of-charge.

The 40-pin DIP 16C550 UART is a device that has historically been used in many communication devices. You may already have one available on-hand - but if not these can still readily be purchased, including from on-line electronics businesses such as Jameco or Future Electronics: P/N 288809 P/N ST16C550CP40-F

The cable to the display only requires 3 wires: Power, Ground and Serial Data. This cable can be self-built from materials on hand or with commonly available parts.

For the serial display unit, the 4-line x 20 character Parallax Model 27979 was initially chosen because of its availability, relatively low cost, large display area and relatively low power consumption. From firmware version 214, the firmware specifically supports the initialization steps for the Parallax 27979. This unit is readily available on-line from companies such as the following: P/N 27979-ND P/N 619-27979-ND P/N 27979

From firmware version 217 onwards, the CC+ can additionally support a SparkFun serial LCD controller. The SparkFun device is a 'controller' and not a complete display - but using it allows a wider selection of the standard HD44780 LCD displays - including ones with alternate colour schemes. The SparkFun controller is available here: Serial Enabled LCD Backpack - SparkFun Electronics.

Note: The SparkFun controller is generic, and will support many different types/sizes of displays and run at different baud rates. The CC+ firmware is written to expect:

Any display unit chosen for use with the SparkFun controller should be able to support 4x20 characters. As well, the SparkFun controller will have to be configured appropriately before use with the CC+.

Per the SparkFun documentation, at least 3 commands will need to be sent to the SparkFun controller before use with the CC+:

A representative red-on-black display which has been tested is HD44780 20x4 characters LCD module Red Character Black background [RT204-1R].

All told, the cost to add on an alphanumeric display, as a do-it-yourself exercise, is under $50.


Installation consists of the following steps:

  1. Install firmware EPROM;
  2. Install display UART; and,
  3. Connect display unit via a cable.

Firmware Installation

An EPROM with version 214 (or higher) should be placed in location U2 on the board, replacing the existing EPROM. The U2 location is highlighted via a yellow box in the picture below:

Display UART Installation

A16C550 UART should be placed in U11, oriented in the same manner as U10 beside it. The notch in the chip should be oriented 'down.' U11 is highlighted via a yellow box in the picture below:

Display Connection via Cable

All internal CC+ cable sets already have the serial display support signals wired to the rear panel of the Chroma on the rear-panel DB-15. Specifically, display power is available on pin 6, display ground on pin 15 and the serial data on pin 14:

To allow connection of the serial display and the MIDI cable at the same time, a DB-15 "Y" cable can be used, such as the one below:

The MIDI cable can connect to one of the branches of the 'Y', leaving the other branch available to connect to a cable going to the serial display.

A custom cable could also be created which only makes the 3 necessary serial wires available for the display, while passing on the other needed wires to the MIDI cable.

The full pinout of the DB-15 on the rear of the Chroma is illustrated via J4 of the internal cable drawing (see also Installation Instructions: Wiring Diagrams for Internal and External Cables):

Internal Wiring Diagram (click for larger view)

For those CC+ users who did not purchase the MIDI/internal cable set (or do not wish to have an external connection for the display), another option exists. Specifically - one of the connectors on the CC+ board is dedicated for the display. This connector can be found by itself, in the lower-left hand corner (J7, labelled as 'Display Conn.', below).

A representative mating connector for J7 is Digikey P/N A28407-ND. The pinout of the J7 connector is shown as part of the internal wiring data (see diagram above).

Whether using the rear panel DB-15 connection, or the internal J7 connection, the intent is the same. Specifically, the Power, Ground and Data signals for the display are to be routed from the connector to the corresponding pins on the Parallax display.

The photo below illustrates the rear of the Parallax display.

The connection to the display is via a push-on 0.100" header. A standard header connector can be used for that portion of the connection.

The current CC+ display implementation runs at 19,200 baud, and thus both SW1 and SW2 should be set to "on."

Expected Output

Once you have installed an appropriate firmware image, the display UART, connected the display unit, and configured it for 19,200 baud you are ready to start using the display. When you power up the system, the display backlight will turn on, the Chroma's auto-tune cycle will complete and the serial display unit will show a decoded version of the parameter represented in the small 'Data Readout' LED display.

For most of the parameters:

The first line of the display will show:

The second line will generally contain the long-text explanation of the value of the parameter.

The third and fourth lines are used to summarize related parameter values. For instance, for any envelope parameter, the envelope's Attack, Decay, Release, Attack Mod and Decay Mod settings will appear.

In the sample output above we can see:

Line 1:

Line 2:

Line 3:

Line 4:

For parameters with multiple modulation sources - all modulation sources and modulation amounts will be displayed:

In addition to parameter data, the display also helps to decode 'set split' and 'link' data:

Voice Card Debug

In addition to viewing the normal parameters of the Chroma, the Alphanumeric Display can also be used to debug failing voice cards by providing specific data on the nature of auto-tune failures. Details on that functionality, and the data expected to be seen on the display, are discussed in The Chroma CPU Plus (CC+): User's Guide - Set Split 40 Display Autotune Failure Information.

User Implementations

David Clarke's Parallax display mounting:


Tom Moravansky's [21030431] display (see thread The Chroma Now Supports an External Alphanumeric Display):


A possible implementation of a 'Y' cable to allow MIDI and the display to be connected at the same time:


SparkFun serial-to-LCD controller with a cheap red-on-black HD44780 20x4 characters LCD module Red Character Black background [RT204-1R] display:


Michele Majorano's BCR2000 with custom end-cheeks and a custom mounting of a SparkFun enabled red-on-black LCD display at the rear.



The Parallax 27979 comes without a case. Users are free to decide how/where the display will mount. Some may choose to create custom enclosures.

The decoded data for Parameter 1 (Patch) and Parameter 3 (Keyboard Algorithm) contains a free-text description of the parameter in question.