Super Mini-Node Interface Card (SMINI) - Part 1

Neither standard includes any of the OSI model layer two, where the bit framing protocol resides. The connector resides in layer one of the OSI model and RS-232 includes the connector. Of its 40 pins, 33 are I/O lines, configured in three 8-bit ports, one 6-bit port and one 3-bit port. These are signals from the PC, signals to the PC and ground. Only one SMINI, as determined by the PC, may be permitted to communicate back to the PC at any point in time. This means that only the U7 transmitter on one particular SMINI can be permitted to be active at a time, otherwise there would be more than one U7 trying to pull the transmission line high or low at the same time resulting in a confused state. If you do, then U6 and U8 both fight to control the RX line resulting in faulty operation and possible IC damage. Some RS-232 to RS-485 converters originally used the handshaking signals such as RTS to control the enable of the RS-485 transmitter. U8, a dual RS232 transmitter and receiver IC, handles communication in both directions when using RS232. Three wires are used with RS232.

If you are using RS232 you simply install U8, or for RS485 you install U6 and U7. When the logic is 1, the device is in the sending state; the A and B terminals are the differential signal terminals for reception and transmission, RS485 standard respectively. RS-485 typically uses a linear network with bidirectional signal wires: There are typically many devices along a RS-485 shared cable. The 8-lines of U1’s Port RD (Pins 19-22 and 27-30) are the bi-directional I/O data bus connecting U1 to the SMINI’s 3 input ports and 6 output ports. RB0 - RB5 used as outputs to drive the 6 latch enable lines for separately activating each of the 6 SMINI output ports. Fig. 4-6 shows a sub-schematic detailing the latch and output transistor portion of the SMINI card, configured for both the standard current-sinking and optional current-sourcing. RE0 used as output to drive the 3rd SMINI input port enable. When it comes to the PC receiving data from the SMINI cards, the situation is more complex. The receiving device on each SMINI, the U6 receiver IC, has a high input impedance corresponding to a ¼ unit load. U6 and U7 are identical RS485 transceiver ICs capable of transmitting and receiving over the same 2-wire transmission line to support half-duplex operation - two wires transmitting bi-directionally.

Half-Duplex: RS-485 typically operates in half-duplex mode, meaning data can be transmitted and received, but not simultaneously. RS-485 signals transmit on a serial bus, meaning all senders (masters or managers) will be detected by all receivers. Then, RX interrupt handler will scan received message via RS485.scanf. Everything focuses on U1, the PIC16F877, so I will cover it first. For example, if U1 needs to write to I/O Card 0 Port A, it first sets Port RD to be output then it places the data for output on the Port RD lines. Likewise, when U1 needs to read from I/O Card 2 Port C, it first sets Port RD to be input then it pulls line RE0, Pin 8, low to activate, or enable U5 and then it reads the input data that U5 places on the Port RD lines. RC0 - RC3 used as inputs to read in the SMINI baud rate DIP switch segments. RA0 - RA5 used as inputs to read in the SMINI Address DIP switch segments A0 - A5. Table 4-2 summarizes the utilization of the 877’s I/O port lines for the SMINI application. The following table provides an overview of the two standards. This provides easier system operation and debugging capability.

The C/MRI is set up for full-duplex operation - two wires for transmitting from the PC and two wires for reception by the PC. 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. With U9 enabled it transfers the data from the Port RD lines, the I/O bus, to the 8-output pins associated with Card 0 Port A that are connected to the railroad. The /RE and DE lines are tied together, when set high the transceiver is in receive mode, when set low, the transceiver is in transmit mode. The arrows indicate the direction of data transmission for each two wire set. Userspace application code normally drives the direction bit of the RS845 transceiver.