MSP430 with RS-422 (Hardware Question)

Nolan Conway

Other Parts Discussed in Thread: MSP430FR5969

Hello,

I am using a msp430fr5969 board. I need to communicate with a Airmar 150wx unit to receive serial strings. The Airmar uses RS-422 communication protocol. If i am correct in interpreting how that works it will send a maximum differential voltage of -6 to +6 volts to the Rx and Tx pins on the board. I think the pins have a maximum input of 4.2 volts but I am not sure.

Can someone confirm whether or not the board Rx and Tx pins could handle signals sent at the levels from Rs-422? If not, does any one have a suggestion of to make this work?

Thanks,

Nolan

over 10 years ago

0 old_cow_yellow over 10 years ago

Guru 58965 points

A msp430fr5969 chip can have Vcc and Vss differential of -0.3V to +4.1V MAX. But for normal operation, this is expected to be between +1.8V and +3.6V.

A board that includes a msp430fr5969 chip may have any voltage on it. It all depends on what other circuits and components are on that particular board and what that particular board is designed for.

0 Nolan Conway over 10 years ago in reply to old_cow_yellow

Prodigy 120 points

I am using the launchpad board (www.ti.com/.../msp-exp430fr5969).

If it can not handle the voltage do you think a simple resistor based voltage divider would suffice?

0 old_cow_yellow over 10 years ago in reply to Nolan Conway

Guru 58965 points

That board cannot handle RS-422 signals.

For RS-422 signals going into MSP430FR5969, resistor dividers can be used. But RS-422 signals in the opposite direction cannot.

0 Nolan Conway over 10 years ago in reply to old_cow_yellow

Prodigy 120 points

So if all i need is to receive data I will be OK? All i want to is serial read the NMEA strings the sensor unit outputs. If I have the Rx side configured with the voltage divider, will I board be able to interpret the rs-422 protocol directly with a call to serial.read() or is more manipulation required?

0 old_cow_yellow over 10 years ago in reply to Nolan Conway

Guru 58965 points

I think a 5k resistor from RS-422 TX+ signal to the RXD pin of MSP430 chip is all you needed. MSP430 has input pin protection diodes that can handle up to 2mA.

Be aware that the data-rate will be very limited.

0 Tony Philipsson over 10 years ago

Guru 12050 points

Output options include:
-RS422/CAN BUS
-RS232/CAN BUS

Looks like it can do RS232 too, and is CAN not TTL levels? so 0v and 5v

3.3V CAN system

http://www.mouser.com/Texas-Instruments/Semiconductors/Interface-ICs/RS-422-RS-485-Interface-IC/_/N-7e3jcZscv7?P=1z0zls6

0 Nolan Conway over 10 years ago in reply to old_cow_yellow

Prodigy 120 points

What do you mean the data will be limited? Do you think 4800 baud rate will be ok?

0 Nolan Conway over 10 years ago in reply to Tony Philipsson

Prodigy 120 points

I know it says it can do Rs232 however it can not actually output that. It follows instead RS422. As for CAN, that only allows you to use NMEA 2000 strings instead of the NMEA 0813 (required for processing).

0 old_cow_yellow over 10 years ago in reply to Nolan Conway

Guru 58965 points

Nolan Conway said:
What do you mean the data will be limited? Do you think 4800 baud rate will be ok?

RS-422 can handle data-rate up to 10 Mb/s. I do not know what will be the limit for this kluge being discussed. But I know it is going to be much slower than 10 Mb/s.

0 Brian Boorman over 10 years ago in reply to Nolan Conway

Guru 25210 points

Do it the right way. Use an RS-422 interface chip between the Launchpad and the external unit. See: www.mouser.com/.../N-6j75f

Trust me. This is cheaper than buying a new launchpad when you accidentally destroy the I/O pin on the MSP430 with a bad voltage level.

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MSP430 with RS-422 (Hardware Question)