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TXDI serial interface board
A TXDI board is used in conjunction with a KNJN development board for RS232 communication.
Background
When the Pluto board was designed, the goal was to create a small FPGA board that could communicate with a PC using the 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.
TXDI is now available in more flavors - with integrated voltage regulators and/or MAX232. This makes TXDI more versatile.
How is it connected on a Pluto/Pluto-II/Pluto-3
TXDI can be plugged directly to an RS-232 connector (directly on the back of a PC, or using an RS-232 extension cable).
TXDI includes the power connector to power Pluto/Pluto-II/Pluto-3. Use a common DC power-adapter that provides something like +9V (anything between +5V and +15V is fine). Make sure the DC-adapter provides the positive voltage in the center of the plug.
New flavors of TXDI
There are currently 3 TXDIs available:
- TXDI: the original version.
- TXDI/MAX232/5V-reg: TXDI + MAX232 + 5V regulator.
- TXDI/MAX232/5V-reg/adj-reg: TXDI + MAX232 + 5V regulator + potentiometer-controlled adjustable regulator.
| TXDI | TXDI/MAX232/5V-reg | TXDI/MAX232/5V-reg/adj-reg | |
|---|---|---|---|
| Power in | 5V to 15V | 7V to 15V | 7V to 15V |
| Power(s) out | 5V to 15V | 5V fixed | 5V fixed 2.6V to 12V adjustable |
| Pluto/Pluto-II/Pluto-3 compatible | Yes | Yes | Yes |
| Dragon/Saxo/Xylo compatible | No | Yes | Yes |
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: TXDI uses 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.
TXDI/MAX232 with Dragon/Saxo/Xylo
TXDI/MAX232 can be used to add an RS232 port to Dragon/Saxo/Xylo.
Note: Dragon/Saxo/Xylo 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.