Instrotech Application : RS485 DIGIBUS

This application note describes how to setup a RS485 bus using the digibus protocol

• The instruments to be used must be ordered with an RS485 option
• Each instrument must be set to a different address BEFORE connection to the bus
• The instruments and the computer must use the same baud
• The Ascii bus menu must be off on all instruments

SMART CONVERTER
The A53 converts RS232 from a computer into RS485 for the bus. This converter is SMART as is changes the RS485 to transmit whenever a message is sent by the computer. No handshaking line RTS is required and no software is needed.

Setup

• Open the converter and set DIP switch
• SW1 is on for RS485 mode
• SW2 is on for automatic handshaking
• Insert JP1 for the end of bus terminator 120 ohms
• Wire 9 to 30 volts dc to terminal 6 - and 7 +
• Turn on the power: the CTS and power leds come on
• Plug in the RJ45 cable from the RS232 connector to the computer
• Wire terminal 1 TXD+B to the instruments D+, all in parallel
• Wire terminal 2 TXD-A to the instruments D-, all in parallel
• To extend the RS232 cable connect white to pin 5 0v, green to pin 3 tx, blue to pin 2 rx : dsub 9 female

Network Test
To test the network with the DOS DIGIBUS.EXE program

• Run the program
• Type in the COM number 0 to 1 = port 1 to 2
• Type in the unit number of the instrument
• Type in the register number eg 8C, 4001 process display
• The program will show you the string sent
• The program will show you the string received
• The program will show you the result

PROTOCOL
Digibus protocol document

• Plug an rs232 cable between the two RS232 ports on your computer
• Run the DIGIREM.EXE program, setup for COM0
• Run the DIGIBUS.EXE program on COM1

• Once you see how this works, quit the digibus program
• Test your program on COM1 and watch the result

If this does not work then lower the BAUD rate