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KNJNelectronic development products

TXDI RS-232 serial interface board

TXDI is an RS-232 interface board and is mainly used in conjunction with a Pluto FPGA board, although new flavors of TXDI can also be used with other boards.


When the Pluto board was designed, the goal was to create a small FPGA board that could communicate with a PC using an RS-232 interface. The problem faced was that a small board could difficultly accommodate a bulky DB-9 RS-232 connector... which is almost as big as Pluto itself!

So the DB-9 was kept outside.

The second problem was that an interface circuit is required for RS-232 communication. A downstream RS-232 interface is simple to design, but an upstream interface usually requires 'voltage-doublers' charge-pumps (like in the famous MAX232). The original TXDI provides a different solution by drawing the RS-232 voltage levels from unused signals on the RS-232 interface itself, so it doesn't require voltage-doublers nor specific power supply voltages.

Flavors of TXDI

TXDI is now available in more flavors - with integrated voltage regulators and/or MAX232.

Power in5V to 15V5V, or 7V to 15V7V to 15V
Power(s) out5V to 15V5V fixed5V fixed
2.6V to 12V adjustable
Pluto / Pluto-II / Pluto-3
Dragon / Saxo / Xylo
Do you need a 5V output?

Having a 5V regulated output is useful as many peripherals (like LCDs) use 5V as power source.
Pluto/Pluto-II/Pluto-3 have their own 3.3V power regulator and work fine with (but don't require) a 5V supply.

Do you need a MAX232?

Short answer: The MAX232 makes TXDI more versatile.

Long answer: The original TXDI uses power from unused lines in the RS-232 cable (specifically RTS and DTR) to convert between RS-232 and TTL signals. That works as long as you have control over these RTS and DTR signals. The FPGA application delivered with Pluto boards has control over these lines and works fine with any TXDI. If you want to use your TXDI in more applications, a MAX232 is more versatile as it doesn't rely on specific levels on RTS and DTR.

In addition, TXDI/MAX232 can be used to add an RS232 port to Dragon or Saxo/Xylo.
Note that these boards have a 'secondary connector' that provides 5V power. The TXDI connects directly to the secondary connector and is powered through it - no need for a DC adapter in this case.

To purchase a board

Go to the Shop - TXDI boards page .