Modifying the CC90-232 interface

This achievement is an original idea of Yves Charriau and François Mouret

CC90 is a 38400 baud serial transfer program (about 4ko per second) between today computers and the Thomson. To make it work right, you need some material :

  • An USB/RS232 adapter (or USB-Serial) you could find it in the shops or on the Internet. Choose a Prolific PL2303 chipset, whose technology is compatible with most of platforms.
  • A CC90-232 interface, that you already have or you will find on the Internet, and that you will have to modify.
  • A DB25/DB9 adapter fully wired (no 'null modem'!).

The little electronic modification won't alter your interface faculties.

Equipment

The CC90-232 interface, as such, can not work properly in serial mode on rare Thomson what have -5 volts supply, in addition to 12 volts.

The designers of the CC90-232 interface used simple components to convert 12 volts into -12 volts. The voltage of -5 volts is used to prevent the power loss due to dissipation.

Nowadays, some components are able to convert a positive voltage into a negative voltage without loss. The deal is to create the -12 volts from the 12 volts supplied by Thomson ... without passing through the -5 volts.

What is necessary:

  • A 8x8 holes pre-drilled stripboard on 0.1 inch (2.54 mm) centers
  • A DIL component that is able to convert the 12 volts into -12 volts (here: a 7660SCPAZ of Intersil)
  • Two capacitors 10µF 35v (as required by the IC... so it depends on the IC)
  • Three wires to connect the assembled interface

Scraping off the stripboard

You must first scrape off four strips in the middle of the stripboard. Use, for example, a retractable blade knife to cut the strips either side of the zone and then scratch the copper with it.

Stripboard copper side Stripboard component side

Soldering the components

A pretty simple wiring. To further facilitate the work, start with straps that you will make with the pin connectors of the two capacitors. Solder then the IC and finally the capacitors. Finally, solder the three wires (here white, green and black, but colours don't matter). Interface side, the white wire is connected to the 12 volts, the black one to the ground and the green one to the well expected -12 volts.

Extract from datasheets Extract from datasheets
PCB layout Component side of the wiring Copper side of the wiring

Be sure to solder the IC straddling the scraped part in the indicated direction.

Testing the circuit

Use an electric battery which not exceeds 12 volts. Connect the white wire to the +, the black wire to the - and check with a multimeter that there is an inverted voltage between the green wire and the - of the battery.

Desoldering the useless components

This operation will allow you to put the voltage converter into the space created once everything is finished. The identification is all the easier given that the name of each component is printed on the circuit board.

Before After
Voltage converter layout

Just remove:

  • T1, T2, T3 and T4
  • R2, R3, R4, R5, R6, R7, R8 and R9
  • C5, C6 and C8
  • D1, D2 and D3

Connecting the interface

Connect the green wire to the + of D3, the black wire to the - of D3, and the white wire to 12 volts (thicker circuit on the left), after reducing the wire to a minimum length. Also check the inclination of the solder so that the wires go in the opposite direction of the ribbon cable.

Wiring of the voltage converter Close up of the wiring

Now wrap your wiring in a thin cardboard to avoid short circuits.

Integration of the voltage converter

Integrating the voltage converter

Place all at the location of the desoldered components and close the interface. Don't forget to replace the insulating sheet between the two parts of printed circuit boards.

Your interface is ready for use.