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Richard Erlacher
12/26/12 14:12
Read: 939 times
Denver, Co

#189035 - Could the data rate be a problem?
Responding to: Mahmood Elnasser's previous message
Mahmood Elnasser said:
I used the MAX483 chip with its Vcc connected to regulated 5V supply for my 7 clocks with a network of around 100 meters. It worked for a couple of years now. Customer expanded his firm and ordered another 7 clocks to be added to the network, 1 on the 5th floor one on the second floor and 5 on the 1st floor where the old 7 clocks are located. Looks like the trip to the 5th floor and back to 2nd floor and back to 1st floor took another 100 meters of length.

I noticed 2 of the 5 clocks at the end of line do not receive the data, I measured the amplitude of the RS422 signal and it was very low less than 0.5v. So I made RS422 repeater board for the last 5 clocks and supplied them with 5Vdc Vcc which solved the problem.

Thinking about the problem again (After it has been solved) I noticed that MAX48x chips are rated for 12Vdc supply. If I raised the Vcc of my master MAX483 chip to 12Vdc, probably I don't need the repeater board to extend the range and would have saved my self one day of work.

What Vcc supply voltage do you use for your RS485/422 applications?

Standard 	EIA RS-485
Physical media 	Twisted pair
Network topology 	Point-to-point, Multi-dropped, Multi-point
Maximum devices 	32 - 256 devices (32 unit loads[1])
Maximum distance 	1200 metres (4000 feet)
Mode of operation 	Differential signaling
Maximum baud rate 	100 kbit/s - 10 Mbit/s
Voltage levels 	        -7 V to +12 V
Mark (1) 	        Positive Voltages (B-A > +200 mV)
Space (0) 	        Negative voltages (B-A < -200 mV)
Available signals 	Tx+/Rx+, Tx-/Rx- (Half Duplex)
                        Tx+, Tx-, Rx+, Rx- (Full Duplex)
Connector types 	Not specified

The supply voltage for your drivers/receivers/transceivers would depend entirely on the manufacturer's spec's. Clearly, if the intended voltage levels exceed 5 volts, the supplies will also have to do that. It's differential signaling, however, and if you measure both Rx+ and Rx-, in differential mode with your 'scope, and still get only 1/2 volt or so, check your receiver spec's, as some will tolerate that low a differential voltage. You are getting the "eye" level, are you not?

I've always had bipolar 5 volts available, though I don't recall every going below GND on my app's, which were always short-haul, unlike yours.

This signaling protocol should easily allow 10 Mbits per second. I'd look for a fault in the cabling or perhaps a faulty connector before making any circuit alterations.


List of 13 messages in thread
RS485 supply voltage      Mahmood Elnasser      12/26/12 12:00      
   Could the data rate be a problem?       Richard Erlacher      12/26/12 14:12      
   5V      Erik Malund      12/26/12 17:17      
   No!      Kai Klaas      12/27/12 06:24      
      Strange!      Mahmood Elnasser      12/27/12 10:56      
         stubs?      Erik Malund      12/27/12 11:38      
            Found it      Mahmood Elnasser      12/27/12 13:35      
               I made $$ on that one      Erik Malund      12/28/12 10:22      
   How to Protect ?      Satish S Munot      01/09/13 23:32      
      Use MAX3430      Oliver Sedlacek      01/10/13 01:49      
         Or the LT1785...      Kai Klaas      01/10/13 05:10      
      MAX485      Satish S Munot      01/10/13 07:43      
         low      Erik Malund      01/11/13 13:41      

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