The New Fuss About RS485 Standard > 자유게시판

When the computer is to read data from the SMINI input ports, it simply tells the PIC16F877, which gathers the data from the 3 input ports, sets it up in the prescribed format, and transmits it bit-by-bit back to the computer. RC6 used as output for sending serial data back to the PC. One RS-485 network can be used for the master to talk to all of the slaves on the network, and a different RS-485 network can be used for all of the slaves to talk back to the master. For example: An RS-422 network has an always active driver and receiver. For each device interface, the driver and receiver are both connected to these two wires. The data is sent down the wires differentially, or when one wire has a high, the other wire has a low and vice-verse. Since RS-485 is half-duplex, it is difficult for the receiver to tell the transmitter to "shut up" when it can't get a byte into the incoming data. If you really just want to get started, skip ahead to SMINI Parts.

Each of the SMINI output ports is configurable for standard current-sinking or alternate current-sourcing. The majority of C/MRI applications use current-sinking whereby railroad devices are activated, i.e. turned on, by the SMINI completing the ground connection. RS422. RS422 is also specified for multi-drop (party-line) applications where only one driver is connected to, and transmits on, a "bus" of up to 10 receivers. If you keep the cable length at 70% or 80% of this limit you should expect the network to work with a true RS-232 driver and receiver. When 5V drivers are used, the driver typically pulls one wire to circuit common, and the other wire to 5V (and vice-verse for the opposite data). Which circuit is used depends on whether you are using RS232, or RS485 (RS422) I/O. If you prefer, all the circuit boards, including the SMINI, can be purchased fully assembled and tested or as complete kits from SLIQ Electronics. If you have purchased your SMINI already assembled and tested, you may skip to Computer Connections. Circuits may be terminated on screw terminals, D-subminiature connectors, or other types of connectors. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices.

There are two other devices in this figure (receiver and transciever) that are connected to the transmission line the same way. There are also 9 separate I/O control wires - typically called port select or enable lines - joining the 877 to each of the 9 I/O buffering functions. There is no graph of cable length vs. RS-232 is only defined for point-to-point connections, so you need a separate cable for each sensor connected to a host CPU. In practice CAT5 or CAT7 cable is often used. Each of the 24 SMINI input lines feature optional input line filtering for maximizing immunity to electrical noise frequently encountered with pulse-type command control systems such as DCC and Railcommand. For SMINI outputs, say Output Port C on Card 0, the PIC16F877 places the desired output data on the 8-wire I/O data bus and then activates the port enable line connected to the enable input on Output Buffer U11. A single 8-wire I/O data bus connects the PIC16F877 to each of the 9 I/O buffering functions. Although all the I/O lines are physically on the same SMINI card it is convenient to talk about them as being separate (I/O) cards within the same (SMINI) card.

By selectively activating only one of these 9 lines at any one time, the 16F877 sets up direct communication to and from the desired port via the single function 8-wire bi-directional I/O data bus. The same design works whether we are using it plugged into the SMINI or the SUSIC and whether we are using RS232, RS422 or RS485 I/O. Besides being more than twenty times faster and much more powerful than the MC68701 used in the original design USIC, the programmed PIC16F877 is about half the cost. As a bare minimum the package needs to include a programming editor and an assembler that turns the user written assembly language code into 1s and 0s for loading into the PIC16F877 Microcontroller. Programming the PIC16F877 FLASH memory requires a special PIC Micro Programmer package. Do not worry about having to do any PIC programming. The arrows in Fig. 4-3 indicate the direction of signal flow, and double lines with a slash and number indicate multiple parallel wires - four, seven or eight in our case.

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