Tuesday, July 30, 2013

TTL To RS-232 DB9 M&F

There is also a write up of this project at ECN Magazine.

The title is not quite correct. Actual supply levels using the TI SN74LVC2T45DCTR range from 1.65 volts to 5.5 volts. Nominal supply voltages are 1.8V to 5V. Signal input levels will vary with the supply voltages. If you use the NXP 74AVC2T45DP chip the inputs will operate from .8 volts to 3.6 volts. However due to differences in packages you cannot use the NXP 74AVC2T45DP on the board as designed. The pins on that chip are .5 mm spacing. The TI chip has .65 mm spacing. Baud rates to 230,400 are possible but I have only tested the board which uses the TI SN74LVC2T45DCTR at speeds up to 115,200.

You can buy the bare boards Male DB9 or Female DB9 for $8.60. The boards come from OSH Park whose motto is "We're fab." And they are. Gold plated.

You can get the full documentation (schematics, parts list, and chip placement) here.

The board outputs full RS-232 levels (+/- 10 volts nominal about +/- 8 volts actual) for TxD, RxD, DSR, DTR, CTS, and RTS. These signals are available in a jumper field allowing you to jumper the board (with jumper shorts) for DTE or DCE use. RI is also available, but you have to wire jumper it according to usage. Since RI is so seldom used these days that should not be a serious impediment.

One point about the documentation. The schematics for the DB-9 Male board and the DB-9 Female board are identical. So is the parts placement. The only difference is how the DB connectors are wired on the PCB.

Building the Board

Install the 22uF, 6.3V capacitors C1, C2, and C8. Install all the .1uF capacitors C7, C9, C10, C11, C12, C13, C15, C17, C19. Install the 4.7uF 16V capacitors C3 and C4. Install the 2.2uF 16V capacitors C5 and C6. Install ICs U1 thru U5. Install the rest of the surface mount parts. Install ground pin GND0. Install the .1" ctrs headers and jumper fields. Install the DB-9 connector.

Update: 18 March 2014

I have modified the boards so that they can be powered from the device they are plugged in to. For that you use JU2. JU1 has been removed. In addition capacitors C3, C4, C5, and C6 have been reduced in capacitance so that the board powers up faster. You can find the documentation for the board at TTL to RS232 DB9 M&F 25 Sept 2013 Documentation. OSH Park has the TTL to RS232 DB9F 25 Sept 2013 available for $8.60 ea. The TTL to RS232 DB9M 25 Sept 2013 is also available for $8.60 ea.

Engineering is the art of making what you want from what you can get at a profit.

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