Hardware Construction

Although a 19" rack would be ideal it is too expensive, The cards have been designed to fit into a rack style configuration. All cards are of the same size. Each card is independent and has its own crystal. Besides the microcontroller, each controller card requires some driving circuitry to interface to layout equipment. To simplify things, the micro and the driver have been put on the same card. While opto-isolation would have been good to have, up to 8 optos may be needed on each controller card which can increase the cost and chip count.

Three power supplies +5V for the TTL logic and the PIC, +12V for the locomotives and relays and +16V for the points. Both the +5V and the +12V supplies have to be well regulated - +5V because the PIC and the TTL chips require one and the +12V because the change on current draw by one locomotive can affect the speed of another if the power supply is unregulated. The +16V is not critical and driving the points with the +12V supply itself will be attempted. The 12V supply may encounter voltage drops due to the driver circuitry. Besides, the controller has a maximum duty cycle of 94%. To compensate for this, a +15V supply will be used. The same voltage can then be used to drive the point motors as well. The ground connections for the +5V supply, the +12V supply and the RS-232 signal will be connected together at one point and will be common thereafter.

To allow programming of the controller cards without pulling out the 16C84 chip, the MCLR connection is also brought out to the connector. This will require an external programming interface card along the lines of the one described in application note 589. This in-circuit programming feature will become indispensable if and when I move to SMT designs. The following 6 connections will go to all controllers cards.

    +5V
    +15V
    Tx
    Rx
    /MCLR
    Gnd

The Tx and Rx connections will go to RB7 and RB6 respectively. This will bring the pins used for programming out on the connector. It may also allow interrupt based input on these pins, if required. The Tx and Rx connection may, in future, be used as I2C connections.

Although the bus topology does allow controller cards to be scattered around, they will be kept in one place. Wires from each driver will go to track equipment. To allow easy connection and disconnection between the controller and the layout, connectors will be used at both ends. Modular connectors commonly used with telephones are used here. The 4 way connector would be preferred one. Modular connectors have the following advantages.

Inexpensive(Plug - AU$ 0.50, PCB socket - AU$ 0.75, Cable - AU$ 0.55/m, Crimping tool AU$ 15)
Cables are easily made. There is no soldering involved or shorts to worry about. A connector is easily crimped on to the cable. However, the PCB sockets are a bit of a pain as they don't fit on a 0.1" matrix.
Connections and disconnections are easily made. Insertion and withdrawal force required is low but connections are positively locked.
With 4 wires, most pieces of equipment can have a single connection going to them.

Each controller card will have one or more modular connectors on one edge of the card. Connections from these will go to layout equipment which will have similar connectors.

For the common connection to each controller card, a back plane arrangement can be used. If they were scattered around, the 6 way modular connector would be the choice. The common bus connector will be on the opposite edge. The connector will most probably will be a 2.54 mm SIL connector with a 90° plug on the card. At the layout end, a small PCB mounted on the layout frame will have a modular connector to receive the cable from the controller card. A PCB mounted terminal connector on the same PCB will have wires going to the equipment.

All controller cards and the power supply are fitted into a plastic case about ??mmx??mmx80mm(H). Controller cards are mounted vertically so that the modular connectors fit into rectangular holes on the front panel. Besides an array of modular connectors from the controller cards, the front panel has an ammeter monitoring the drain on the +15V supply. The controller cards are mounted on the front panel using L brackets.

The rear panel has an IEC mains socket and a switch with a built in indicator. A 9 pin male D connector on the back panel takes the RS-232 connection from the host.

The rear half of the case contains the power supply transformer and the power supply card. The transformer has connection for the +5V and the +15V supplies. The power supply card also has a connector gong to the 9 pin D. It also has a series of 2.54mm headers to connect to controllers.

Updated on 2 Nov 95. Feedback to