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i made a circuit for communication between microcontrollers..1 master / multiple slaves and full duplex. please can you guys have a look on it , and tell me where i made mistakes. i know im missing some resistors also but really dnt know about them.so please guide me
The circuit is basically OK. Dependending on cable length you should have 120 ohms parallel (A to B) termination at the receiver. The Slave to Master channel, that hasn't a transmitter enabled permanently, should have weak pullup/-down resistors to set the line level to defined idle state (A to VCC, B to GND). Additionally, you may use a combined full-duplex receiver/transmitter chip, e. g. TI SN65HVD3083E instead of two MAX485 or similar.
Regarding the original topic question, there are mainly differencs in speed and power consumption. My standard chip is SN75LBC176, that is fast (10 MBps) and also low power. Standard SN75176 has unpleasant high idle supply current, but is very cheap.
Hi,
1.You can keep RE- permanently grounded and use logic control only for transmission on slave side. On the Master side, you can permanently enable both the transmitter and the receiver.
2. For such 4-wire applications, you can also use MAX 488 to MAX 491 ICS depending up on your application and cost considerations.
what about termination resistance my last circuit?? actually im not getting the termination concept...can you tell me where i have to place them in last circuit?
really im confused about termination
and about protocol..... can i implement CAN here... i.e using physical layer of rs485 ?
With fullduplex (one-way transmission with transmitter always enabled), you need a termination resistor at the receiver-side line end. That also implies,you shouldn't have long stubs connected to the line, if multiple receivers are connected, the line should pass each receiver without a stub, if possible. Full duplex line must be terminated at both ends.
CAN uses a driver hardware different from RS485, although it also works on differential pair.
I don't know, how SMBus should be implemented on RS485, it isn't UART based at all. Protocol should follow on your communication requirements, but I didn't hear about this point except for the multipoint aspect. So device adressing is apparently necessary. Additionally, you may either have a data addressing for the nodes or you can use free formatted telegrams. If data addressing is suitable, MODBUS could be a simple protocol, that would allow to access single and multiple registers and bits. Also the device addressing is defined.
thank u u Fvm....
im thinking to go with CAN, i know something like this
master>CAN controller>CAN transceiver>----- CAN bus-----
is that right configuration? also when doing CAN is it better to implement CAN protocol in software
or making use of CAN stand alone controller?
please guide me!!
btw i implemented a very simple protocol for 1 master 3 slaves and its working in full duplex
but need is to implement some standard protocol.
hello ,
i want to implement modbus protocol over rs485 layer in full duplex mode. i have at89c51 microcontroller as master and it has 25 slaves(at89c51). i have read modbus specification manual.and i wish to implement Modbus ASCII.
i dont wana implement all functions. i want to go with following functions
1>read coil status
2> read input status
3> read holding registers
4> Read Input Registers
5> Force Single Coil
6>Preset Single Register
i just need some sample code in c language.or some sample algorithm as reference. or if anyone who have implemented modebus please guide me.
i just need starting point and some programming guideline.
so please help me doing this!!
any help will be appreciated!
Thankx in advance...
Looking for some response
Added after 57 minutes:
please help frends...i know lot of people on this forum have worked on Modbus
you can get all necessary information for implementing modbus protocol from The Modbus Organization. CCS has made drivers for modbus protocol and it would be benificial for you to study the provided driver file before implementing your own.
I have implemented the modbus protocol and working properly but that is for PIC microcontroller using CCS C compiler and for AT89C51 you may be using uVision Keil IDE therefore study the protocole in detail and then implement it.
i think for me the value for money IC is SN65NVD3082 from texas.
MAX 485 has problem with ESD and long cables installation with big noise.
SN75176 is vey old IC.
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