Rs485

10gigbill

RS485 question:
I have a system with 10 props communicating over rs485 using the 75176 chip and a 2n2222 transistor as a level shifter between the prop and the control/transmit pins. This has been working fine for the last year at 9600 b. but now I’m looking at the design and thinking maybe I don’t need the level shifter and maybe I should be using a different chip. (like the LTC1487). or?...
Also would like to up my baud rate to maybe 56,000.
A little circuit design help would really be appreciated.

Bill

Phil Pilgrim (PhiPi)

DigiKey is your friend in situations like this. Go to http://www.digikey.com and type "RS485" into the search box. From there, it's just multiple choice through the range of parameters, power consumption, packages, and pricing.

-Phil

LoopyByteloose

How are you thinking of removing the level shifter? Are you looking for a 3.3v RS485 driver instead of what you are using?

I often look through Futurlec.com for what is available and cheap because they provide all the PDFs next to the component description and prices. It is at least a good starting point. Linear Technology and Maxim have RS485 line drivers and transceivers in many different specs.

The MAX3362 is a 3.3v RS485 transceiver. So is the MAX3485 at $2.90 from Futurlec.

10gigbill

Thanks guys.
When I originally designed the circuit I used the 75176 as it was kind of the ‘standard’ chip and I made the assumption that it was TTL on the interface side (and a 5v device), so I put in the level shifter transistor. But now after reviewing the data sheets for the old chip and some of the newer ones I see the 5v chips all have 2 volts minimum for a high on the interface side. I have 5v regulated on the board also, so it’s just the prop to transceiver interface in question. Before I order another hundred chips and re-design the board maybe someone has been there done that.

Bill

Peter Jakacki

RS485 with the Prop is even simpler as you only need two I/O lines. On other micros they have dedicated UARTS with dedicated pins but the Prop is far more powerful than this. I've grabbed a section from one of my circuits just to show you what you should do. Bear in mind that while this is a suggestion this is also the way I do it in commercial datacomms grade equipment so you know it will work, and work well.

I prefer to run either the basic MAX485 or generic equivalent (or even some of the better parts which have proper failsafe inputs, but more later) at +5V for greater drive and range. Failsafe inputs on some just means if you leave the lines floating then you are guaranteed an inactive condition on the logic receive line. Proper failsafe ensures that even if the lines are shorted or as is more frequently the case overloaded with termination resistors then the receive will remain inactive. More on this later.

Anyway, this is the circuit and as you can see I always use a pulldown on the TE line which is also tied to the /RE line as well. So in power-up or undefined state the transmit section will be disabled.The 10K resistor in the combined RO/DI line just ensures the current is limited into the Prop when receiving.

Anyway I might run a design tip thread just on RS485 along with some commercial-grade code suitable for the OBEX.

10gigbill

Peter :

Thanks a bunch! That’s just what I was looking for. My background is power engineering; That is: megawatts and kilo-volts, so microchip interfacing is a little different. I have had a lot of success with the rs485 communications and will probably go with one of the newer chips with static protection and the other extras… Thank you for your support.

Bill

Peter Jakacki

Try this part as well, the SN65HVD3082ED which is 1/8 loading and full fail-safe and at a good price too.