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I just cannot overemphasize the importance of keeping your software simple, straightforward and easy to understand! However, I have included RN12 for two reasons: it is required for the current-sourcing option and keeping it in place for the current-sinking option assists debugging by allowing easy determination of whether an output line failure is within the latch or the output transistor. These transistors, typically 2N4401 NPN transistors, operate in an open collector configuration to provide a sinking capability of .25A for loads up to 40Vdc. Railroad loads such as lamps, LEDs, relays, and switch motors are turned on and off by your PC software turning each output transistor on (transistor conducting with collector connected to ground) or off (transistor open circuited). Table 4-3 summarizes the logical relationship between latch output and the connected railroad device, both when it has been turned on (activated) and turned off (deactivated). When the C/MRI application software, resident in the PC, requests that a given SMINI node send the railroad data to be received as inputs on its 3 input ports, the U1 gathers the data, converts it to serial format, then activates U7’s DE enable line, which then seizes the serial bus for sending the input data to the PC.

Lines RB0-RB7 and RE0 perform as port enable lines to define which I/O port IC (U3-U5 for input or U9-U14 for output) is activated for each input and output operation over the I/O data bus connecting all the ICs to U1’s Port RD. The circuit functioned normally over the entire temperature range. The 974B-72034 QuadMag Cold Cathode / MicroPirani / Piezo Transducer offers a measurement range from 1x10-8 to 1500 Torr and features a 2.75 inch CF flange fitting. RS-485 stands out from other signaling standards due to its significant differential output voltage and extensive common-mode range. This in turn means that the drivers' heat dissipation increases significantly (more current and a greater voltage drop across the output transistors), which is certainly a safety risk. Including the pull-up resistor is mandatory for the current-sourcing case to insure that the base voltage of the transistor is pulled sufficiently high when the latch is high so that the transistor is turned fully off. When the output of the latch goes low the base of the 2N4403 transistor falls below the emitter, causing the transistor to conduct to activate the load. For RS485, the impedance of just 70 Ω may prove a tad too low for longer distances.

However, you may find situations where you need to connect the interface to external hardware that is prewired with a common ground and what you need to do is switch the hot, or power side, of the loads. 5V from the master device (rather than set up a separate power supply). The maximum device handling capacity of the serial standard protocol RS485 is 32 nodes on a single network segment. What is the maximum device handling capacity of serial standard protocol rs485? This packet includes a unique device address, a command or instruction, and any necessary data. RS485 is a half-duplex communication protocol, which means that data can only be transmitted in one direction at a time. If you do not have either one of these situations you can simply skip ahead to Input Line Filtering. Thus, if you are not currently interested in implementing input line filtering, simply skip ahead to SMINI Parts.

Then later, if you find it desirable to add filtering, it is a simple task to cut the trace on the bottom side of the board and insert the desired parts. Simply cut the narrower circuit trace located on the bottom side of the card under each of the resistor locations, and install the corresponding R6-R29 resistors and C24-C47 capacitors. It performs as if the computer interface were a large collection of toggle switches, where one side of each switch is connected to ground and the other side is available for connecting to external hardware. For the system to work, only the transmitter IC from one SMINI can be allowed to seize, i.e. transmit over, the bus at any one time and all other SMINI cards must have their U7 ICs held in their high impedance (open circuited) states. For this to happen without interference, the outputs from the U7 transmit ICs on all the other SMINI cards must be held in a high impedance state, effectively an open circuit to prevent loading down the bus.

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