problem with rs485 1 master 2 slave

[talamahahahe]

i was communicate rs485, transfer data between 2 pic 16f887 ok (1 master and 1 slave) but now when i use 1 pic master 2 pic slave i can't send data to both of slave.

i use ccs 5.048

this is code for master

[Ttelmah]

Describe accurately (or post a drawing), of the RS485 connections.

Wires to PIC.
Which pin on the driver chip connects to which pin on the PIC's.
What bus termination you have?.
What bus biasing you have?.
What pins from each driver connects to the other?.

[temtronic]

tips.
1) put a delay_ms(500); before the lcd_init(); call. This will allow the LCD module to properly powerup before you access it.

2) In the 'master' program put a delay after sending each message. Though I haven't looked at the 485 driver, it may need some 'time' between each sending to the slaves.

3) In the 'master' pgm, I'd put a 'local LCD' confirm that you do send the msgs,at least for test purposes.

Since you say it works for either slave 11x or 12x but not both, I suspect it's a 'timing' issue (software) and not hardware(unless you have to move wires)

Jay

[Ttelmah]

I think he is not controlling the RS485 direction properly.

So the slaves don't deselect. This wouldn't 'matter' for one slave, but for multiple.....

[talamahahahe]

i am sorry to reply late, i was busy at school few day. This is schematic i simulate in proteus 8.1 . Maybe it conflict in vie because i see in proteus , some signal pin of lcd have yellow not red or blue. i don't know i wrong hardware or software.



i was try on delay_ms(100) after send message, but it doesn't work too,

this is all of my simple project, please help me

[talamahahahe]

ex_rs485.c in example just have communicate rs485 with pic and computer. Do you have sample code communicate 1 pic master and 2 pic slave, maybe i will understand clearly if i read sample code

[temtronic]

Have to ask are you using REAL hardware ?
The schematic you show from Proteus will NEVER work in the REAL World.
You require proper RS485 termination resistors( pullup,bias,pulldown).

There are several 'threads' about this ,as well, just google RS485 to get a site with the specs, or just download the datasheet for the RS485 transceivers ( which are NOT in the 'schematic' !!).

Jay

[talamahahahe]

i just test in proteus ,when it simulate ok, i will do real board, just 1 thing, master just can send data to slave 1 or slave 2 , can't send for 2 slave simultaneously. i was do every thing to do that but failure

this is schematic i use max487 to communicate rs485, either slave 1 receive data or slave 2 receive data, can't both of them



with master code

[temtronic]

You need to read and study RS485 design. The MAX487 datasheet would be a good start.
ie: you need ONE set of 'termination' resistors consisting of a pullup from Vcc to 'A', a bias from 'A' to 'B' and a pulldown from 'B' to ground.
Your schematic doesn't show the pullup and there are too many bias and pulldown resistors.
also normally to tie DE and _RE together and connect to the 'control' pin as designated in the #use RS232...options.....). Just see the example CCS supplies in the examples folder.


Jay

[talamahahahe]

i was use pullup up A to VCC like this. But it now both of 2 lcd show nothing. :( i don't know what problem



both of slave is not working.

But one thing is very stranger when i not use resistor pull up at A point

first, slave 1 will receive data immediately, and after long time , slave 2 receive data (i wait about 1 hour). what problem with it ??

[temtronic]

Are you using real Pics or just ISIS ??

Jay

[Ttelmah]

There also _must_ be a pullup resistor on the line from RD to RX. Otherwise when the bus is being driven, the PIC will receive garbage.

[drolleman]

Why are you dealing with this guy. Proteus doesn't work. Point to the top of the forum, and shut the thread down. This will stop others from trying to do the same. Your talents (amazed by them) are better spent on issues where people are putting the effort.

[talamahahahe]

i don't know which hardware is true so i just simulate on proteus and then ,if everything ok, i will do in real board, can you give me your true hardware, the way you connect pic and max487

[Ttelmah]

1) The first thing to understand, is that Proteus/Isis, is badly flawed when dealing with PIC's. You can put designs into this that refuse to work, yet will run fine in a real chip. You can put designs into this that it says 'yes this works', which do not have a hope of running in a real chip.
Time spent using the Proteus simulator with PIC's, is time wasted. The quicker you understand this, the quicker your designs will start to work....

2) Then the second thing is the 485 bus itself.

This requires two separate things.

First termination. The bus wants to be _linear_. Running from one device at an end, along past other devices (with short 'stub' connections to these - no more than a couple of inches long), and ending with another end device. Each _end_ device needs termination, nowhere else. A lot of commercial systems use a 'through' connector at each device along the bus, and then you plug a terminator into the unused connections at each end. There should be just two terminators.

Then bias. The bus wants bias at just _one_ point. Again the devices with the plug in terminators usually have a jumper/switch on these to enable the bias. You have to be very careful about the direction of the bias. Unfortunately different manufacturers use different names for the bus lines. The standard is the Maxim way, with line 'A' being the un-inverted line, and line 'B' being the inverted line. With this, line A wants to be drawn +ve, and line B -ve. However if using any other make of transceiver, make sure that it is the un-inverted line that is taken +ve. Even better, use transceivers that do not require bias.

The setup should be:

First 'bias' terminator. 750R to +5v. Line A. 150R to line B. 750R to 0v.
The bias resistors are seen as across the impedance matching resistor, so this has to increase on the terminator with the bias, or a mismatch will occur.

Then the bus......

The at the other end, the second 'non-bias' terminator. 120R between Line A and Line B only.

This is for a cable of 120R impedance.

As drawn at the moment, your bus has much too much bias, and too many terminators.

3) Then the final thing is driver and it's connections to the PIC.

RO to PIC RX. _With a 4K7 pull up resistor to 5v_.
This resistor is essential. Without it, when the receiver is disabled, the PIC _will_ see random data. Uurgh.

DI to PIC TX.

Then there are two options. Separate control of DE and RE (pointless in this application), or combined control of these.

For combined control, connect RE on the driver to DE, and this single connection to a pin on the PIC.

Then setup the driver with: