Electronic – MAX485 Burns after working for some time. RS485

In my [in]experience, 99% of serial communication errors are due to timing mismatches; the other* 99% is due to wiring hoopla.

• It works for short wire but not a 30ft CAT-5 cable
• It works for normal serial wire upto 10ft but not more than that.

CAT-5 cable is twisted pair (also, all of what I have used is also shielded, but apparently this is not common); I assume the "normal serial wire" is two loose wires. As wiring gets longer, your nice digital rise times stretch out, effectively shrinking the available window for sampling incoming data. Twisted pair helps this a bit. What this really means is that your UART clock must be prescaled more accurately to the baud rate being used. System clocks for perfect USART communications should be multiples of 1.8432MHz. Short-range communications can tolerate a few percentage error, but as your sampling window shrinks it must be more accurate. What your prescaler (int), and baud error?

prescaler = (int)(F_CPU / (USART_baudrate * 16) - 1)
actual_baudrate = F_CPU / (prescaler * 16)
baud_error = 100% * (1 - actual_baudrate / USART_baudrate)

• It works for 6V battery but not for 12 volt battery or a 12V adapter.

This sounds fishy! This may be a completely different problem, perhaps causing your widget to reset due to a dirty supply or poor regulation. There are stability issues with LDO linear regulators and varying capacitive loads; there may be dissipation issues from a 12V supply; if one isn't using a LDO regulator, then a 6V battery will really be giving you 4.5V to 5V, and won't be well regulated. Power supply issues are in a class all their own, and must be rectified before peripherals can be effectively debugged.

Other than this, there is always the issue with environmental interference, or EMI. RS-485 is made for this, but should be used with twisted pair[, shielded wire,] like the CAT-5 cable you've used.

**(What, my math is wrong? Do you know who I am!? ;)*

You are missing something major. You CANNOT connect a serial connection to an ethernet port and get anything useful out of it.

You will need an adapter from RJ45 to an RS232 conector (usually Sub D9) or to an RS485 connector (Not sure what this looks like.) You will then need the appropriate interface on your PC - that would be a serial port for the RS232, or a special interface for the RS485. Both of these can be had as external units that connect to your PC via USB.

You will then need a VI for Labview that can use the adapter. RS232 should be standard, I don't know about RS485. You can probably purchase an adapter from National that will include the correct VI and or drivers.

Both manuals include information on how to wire the RJ45 to a serial cable. You will have to either build them yourself or try to order them from the equipment manufacturer.