Question about converting RS232 to TTL

[gk20]

I needed to connect a Garmin GPS 18x to a VEX Cortex's UART port, but I discovered that I would first need to convert the GPSs RS-232 signal to a TTL signal for the UART. I found this board and was wondering if anyone could tell me if this is what I need: https://www.sparkfun.com/products/11189

Also, could someone help me figure out which wire would go where on the board? The datasheet that Sparkfun provides isn't specifically for this board and the labels aren't very specific.

[Mike Bortfeldt]

gk20,

Looking at the data sheets and documentation for the devices specified, I think the MAX3232 board should work. The connections (based on my interpertation of the data sheets) will be as follows:

Connect the following 3 pins/wires together
- Garmin Wire #2 (red) vin
- MAX3232 3V-5.5V
- Cortex +5V

Connect the following 4 pins/wires together
- Garmin Wire #3 (blk) ground
- Garmin Wire #5 (blk) ground
- MAX3232 GND
- Cortex GROUND

Connect the following wire/pin pairs
- Garmin Wire #4 (wht) TXD (transmit data) to MAX R1IN
- Garmin Wire #6 (grn) RCV (receive data) to MAX T1OUT
- Cortex Data In (Rx) to MAX R1OUT
- Cortex Data Out (Tx) to MAX T1IN

The 5V from the Cortex will power both the MAX and the Garmin IO (requires an external power source)

Try to verify this wiring before powering the devices as I may be in error. The following locations are where I found the various spec sheets.

Garmin http://static.garmincdn.com/pumac/GP...Tech_Specs.pdf
Cortex http://content.vexrobotics.com/docs/...ev1-PINOUT.PDF
MAX3232 Breakout Board https://www.sparkfun.com/products/11189
MAX3232 Schmatic http://dlnmh9ip6v2uc.cloudfront.net/...eakout_v01.pdf
SP3232EP Chip http://dlnmh9ip6v2uc.cloudfront.net/...3232EBCA-L.pdf

Mike

[yash101]

Excuse my kinda curious nature, but I debug my RasPi using a serial UART connection. I just wire the TX and RX backwards and it works. What is the difference between UART and TTL? Aren't they a similar protocol?

[one_each]

Originally Posted by yash101

Excuse my kinda curious nature, but I debug my RasPi using a serial UART connection. I just wire the TX and RX backwards and it works. What is the difference between UART and TTL? Aren't they a similar protocol?

The RasPi and VEX Cortex's UART are using the same voltage levels (3.3V). The GPS is using RS-232 which runs at 12V and would fry the VEX. The MAX3232 chip converts the voltage levels so the two can talk without damaging anything.

[apalrd]

Just to be clear:

UART is 'Universal Asynchronous Receiver Transmitter' and is a hardware peripheral on microcontrollers used to implement a bit-serial interface. It is essentially the hardware that serializes and deserializes bits from a serial bus into bytes, and handles clocking. It by itself has no voltage, that is dependent on the implementation.

TTL is 'Transistor-Transistor Logic'. Usually this is defined so 0-0.8v is logic 0 and 2.2-5v is logic 1, so 3.3v and 5v are both acceptable voltages although some 3.3v devices do not tolerate 5v. TTL is a digital signal spec, it does not define what the digital signal is.

RS-232 is a standard which uses +-12v as logic 1 and 0. It defines the signal voltages, timing, and connector. The timing can be generated by a UART.

RS-422, RS-485, LIN, and several other physical layer standards also use UART with a different physical transceiver. The signaling voltages are different, but the timing is the same.