Bi-directional rs232 link.

[Allan]

Hi everybody,

I need advice on setting up a bi-directional rs232 link.

I'm using a 18f2321 with v 4.104 compiler software. I'm using the UART hardware so the MULTI-MASTER mode is not available. I'm using National 96176 rs485 chips.

My current program seems to be reading part of the transmitted data back. The INT_RDA interrupt fires when I transmit data, even with the receivers ability to transmit commented out. The rs485 IC's were just changed.

From the #USE rs232 page of the manual:

========================================

SYNC_SLAVE Makes the RS232 line a synchronous slave,
making the receive pin a clock in, and the data
pin the data in/out.

SYNC_MASTER Makes the RS232 line a synchronous master,
making the receive pin a clock out, and the
data pin the data in/out.

SYNC_MATER_CONT Makes the RS232 line a synchronous master
mode in continuous receive mode. The
receive pin is set as a clock out,

========================================
Do I make one end a SYNC _MASTER and the other a SYNC_SLAVE?

- OR -

Can I make both ends at SYNC_SLAVE until an interrupt happens, switch to SYNC_MASTER mode to transmit, and then return to SYNC_SLAVE mode to wait for the next interrupt?

Since the hardware at both ends of the link will be exactly the same it would be nice if I could used the same software. There will never be more that 2 units on the link so I'm hoping I can avoid the more complex techniques of the sample programs.


Thanks, Al

[Ttelmah]

Your thread is entitled 'RS232', yet you are using RS485....
Not a good start.

Now, you need to understand, that RS232, and RS485, are electrical signalling standards. Over both these standards (or TTL), you can send either asynchronous data, or synchronous data. Which are you wanting to send?.
I'd guess, that you want to send async data. 99+% of RS485 busses use this. In which case you do _not_ want to select the sync bits. To handle 'sync', you would need to be sending a permanent 'clock' connnection from the master, the data bus itself (another wire, with reversible data direction), and a third data direction control wire. Unless this is what you really want to do, the sync selections are not used.

What you should be looking at, is the 'ENABLE=pin' option in the use RS232 statement. On the DS96176, connect the /RE, and DE pins together, and connect both of these, to the pin you select as the 'enable' pin on the PIC. Then the transceiver is automatically switched to 'transmit', when data is being sent, and to receive the rest of the time.
Then you _must_ bias the bus so that when nothing is sending, it idles to an 'off' state. A search here will find a lot about this. Basically though you need to ensure that the voltage A-B, on the bus is slightly greater than 0.2v, when nothing is driving the bus. You may need to terminate the bus as well, but this is only 'required', if the bus is long, or data rates are high, and also to help reduce noise on the lines.
Finally, add a pull up resistor on the line from the RO pin to the PIC's RX pin. 10KR or similar.

Best Wishes

[Allan]

[jbmiller]

The 'simplest' solution , since there are only ever two units on the com bus, is to have a 'master' and a 'slave'. CCS has demo code in the examples files...makes a good starting point.
If you want both units to be either master or slave, that opens a whole can of worms...requiring timeouts, bus mastering, collision advoidance, error checking, etc.

For over 25 years I've run remote SCADA systems using the 'simple' solution, on a true single wire bus for over 20 miles to the remotes. Noise has never been an issue, no bus arbitration problems,etc.

[Allan]

I need to clarify something in my post.


http://www.ccsinfo.com/forum/viewtopic.php?t=42046


Ttelmah already gave me the program I asked for (Thanks!) The real problem must be in my modifications to his program.

Knowing the hardware is good I can now sweat over the software.

Thanks again.

Al

[Ttelmah]

[Allan]

[jbmiller]

gee Ttelmah, please give CCS a break. They're just using the convention of saying 'RS232' as the common standard for serial communications. Sure it's 'wrong' but everyone understands it. Like saying 'I need a xerox, when really it's a photocopy, or give me a kleenex when really it's a [spam] tissue.
I've been in this trade since teletypes were the 'man machine interface', egads even teletype was the NAME of the company that made the ASR-33 machines. Back then it was 20mA current loops,60 if you had some real distance to go! There weren't any UARTs yet either,had to roll your own with 7400 series chips. Man, that was the early 70's....shoot 40 years ago......the good old dayze !

Jay

[Allan]

[bkamen]