This is a TMS9918A-based video card for the RC2014. It allows the RC2014 to produce NTSC composite graphics using a classic chip of the 1980s. The TMS9918A was used in the TI-99/4A, MSX, ColecoVision, and Sega SG-1000. Enhanced derivatives were used in later MSX computers, the Sega Master System, and the Sega Genesis.

This board is based on a circuit described by Tom LeMense for interfacing the TMS9918A with SRAM. I started with his schematics, added port address decoding and laid out the circuit on an RC2014 module.

This board can be configured to use the same ports as the MSX or ColecoVision, so it's possible to run some software from these systems, such as ColecoVision games and MSX-BASIC.

Ready-to-Assemble Kits are available on Tindie. These are sold by Michael Kamprath with my permission. However, I offer no warranty or guarantee of support.

Gerbers for the PCB are available. I highly recommend JLCPCB for PCB fabrication. Alternatively, boards can be ordered from the shared project on OSH Park.

Refer to the schematic, the picture below, and the bill of materials below for assembly guidance.

Aside from the board and the TMS9918A itself, all parts are available from Mouser and probably other suppliers as well.

The default jumper configurations for the RC2014 are below.

See jumper description at the Hackaday.io article for more detail on each jumper.

• J4 configures address bits 7-5 which lets you select a block of 32 addresses: 00-1F (left) ... E0-FF (right). For ColecoVision, you would set this to A0-BF (6th position). For MSX, you'd set it to 80-9F (5th position). For Sord M5, you'd set it to 00-1F (1st position).
• J6 configures address bit 4. There are 3 options: ignore (left), 0 (middle), or 1 (right). This lets you use the entire 32 address range, the lower half, or the upper half, respectively. For ColecoVision, you would set this to ignore (left). For MSX and Sord, you would set it to 1 (right).
• JP1 configures address bit 2 and 1. In the upper position, they must both be 0. In the lower position, they are ignored. For MSX, you would set this to the upper position. For ColecoVision and Sord, you would set it to the lower position.
• JP2 configures address bit 3. In the upper position, it must be 1. In the lower position, it is ignored. For MSX, you would set this to the upper position. For ColecoVision or Sord, you would set it to the lower position.

The jumper JP4 is used to determine where the TMS9918A interrupt signal to either INT (upper position) or NMI (lower position) on the RC2014 bus. ColecoVision connects the video interrupt to NMI, so this change was necessary for compatibility. MSX uses INT, and most other systems do as well.

Warning: It appears that the TMS9918A does not have an open collector interrupt output. If you have other cards that make use of the /INT line, such as the SIO card that comes with the RC2014 it's possible the TMS9918A will fight with other chips for control of the interrupt line, which will prevent proper operation and could potentially damage both chips. For safety, it is recommended to use NMI instead of INT in this case. REV4 of the board adds a diode to prevent this.

The optional J7 header has pins (from left to right) for RC2014 CLK1 (REV4 only), CPUCLK, GROMCLK, EXTVDP, and GND. These pins can be used as follows:

• CPUCLK and GROMCLK provide 3.58MHz and 447 kHz clock signals, respectively. A jumper block can be placed on the RC2014 CLK1 and CPUCLK pins to use the TMS9918A's clock output as the RC2014's system clock (be sure to remove other clock sources if you do this). The clock signals could also be used by other boards via jumper cables.

• It should also be possible to daisy-chain multiple TMS9918A chips using the EXTVDP signal, or to genlock an external video source. The extra GND pin can be used with an external video source if needed.

• TMS9918 Manual
• TI VDP Programmer's Guide
• More TMS9918 Documents
• Discussion Thread
• Hackaday Page
• Example Assembly Programs

• SN76489 Sound Card: my SN76489 sound card for the RC2014
• Game Controller Card: my Atari/Coleco/Sega-compatible controller interface for the RC2014
• z80ctrl: my AVR-based bus monitor and bootloader board for the RC2014

Copyright 2018 J.B. Langston

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