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What's this level shifting IC?

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edeca

Active Member
Can anybody tell me what the level shifting IC is in this schematic, or an equivalent so I can pick some up?

Datasheet (and original product).

It's not that I'm too cheap to buy, I might actually pick one up, but this looks like a small cheerful way to shift levels in my own circuits so being able to reproduce would be quite cool. I know that the correct way to do this is with a MAX232 (or equivalent), but if this works with modern serial ports then it should satisfy my requirements.

Bonus points if anybody can explain how it works. Obviously the TX/RX signal is tied to the base of the transistor which enables the switching, but I'm curious how it shifts the levels. I am assuming the resistor between base and collector has a lot to do with it.
 

MikeMl

Well-Known Member
Most Helpful Member
RS-In swings to a voltage below ground; this charges C1 to that negative voltage (minus one diode drop), creating a negative power supply.

RS-In, when move than about +1V turns on the inverter, causing a low to be output at Tx.

RS-In, when below about 1V turns off the inverter, allowing R1 to pull Tx high.

Rx, when near Vcc, turns off the other inverter, causing RS-Out to be pulled down to the voltage stored in C1 (see above).

Rx, when near ground, turns on the other inverter, causing RS-Out to be pulled to near Vcc.
 

edeca

Active Member
Awesome explanation, thanks! Now does anybody know a part number? :)
 

Speakerguy

Active Member
All above are useful tools, but if you need high speed in circuit level translation something like the TXB0104 from TI is a good part (great for SPI buses in systems with multiple logic levels).
 

edeca

Active Member
Thanks for all the part numbers. The DS275 looks like a nice part, so either that or a MAX232A will probably do. I guess it is best to do it "properly", perhaps I'll go with the DS275 for the lower pinout.

I found this whilst searching, maybe it would be better. Only $230 for a board with a MAX232 and a DB9 connector :eek:

Honeywell E055 RS232 MODULE (PCB) - asecurityonestopshop
 

MikeMl

Well-Known Member
Most Helpful Member
To do it on the cheap, use two 2n3904 NPN transistors and four resistors in place of the dual inverter ic.
 
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