HW Design Question MAX232 w/USB to Serial

[Ron]

Hello,

I am hoping someone might be able to answer this.

I usually place a MAX232 chip in my designs for RS232 communication to a PC. Since Serial ports are not often found on new computers I have been using a USB to serial converter cable to communicate with a PC.

If I am using a USB to Serial converter cable is the MAX232 necessary? It would be nice to save a couple of dollars and have smaller PCB, take DB9 pin 2 and 3 straight to the PIC.

If I do not need the MAX232, would it be wise to place a resistor on RX/TX lines between DP9 and PIC? If yes, what value/size would be recommended?

Thank you,

Ron

[Benj]

Hello Ron,

It depends on what your USB to Serial converter is outputting at. If it is a TTL level converter then you do not need the MAX chip. Otherwise if the converter is a true RS232 port converter then you will need to use the MAX chip to convert the +/- 12V back into +5 and 0V.

I have seen some example designs using resistors and diodes and this seems to work well but will not be as bullet proof as the MAX implementation.

Why not go the whole hog and use a USB enabled PIC to cut out the middle man of both the USB converter and the MAX chip? This would be my optimum solution.

[Ron]

Hello Ben,

I have attached a schematic supplied by the company. Does this help you determine if it is eliminates the need for the MAX232?

Thank you,

Ron

[Benj]

Hello Ron,

Yes you need the MAX chip still. The SP213EH is doing the same job as your max chip and converting TTL into +/- 12V.

This schematic linked on this page demonstrates connecting RS232 signals without the MAX chip. As I say this will not be as bullet proof as a MAX chip.
http://www.qsl.net/yo5ofh/pic/lcd_termi ... rminal.htm

[tmd_63]

FTDIchip do a nice chip and even a cable.
If you look at ftdichip.com for the TTL-232RL cable, you can connect this directly into your PIC but be aware, the TX line from the cable goes to RX on the PIC and vis-versa.
You could also use the chip itself and design the circuit for USB with an RS232 option.