Page 1: 8535 Obviously the main component on this page is the microcontroller. It has an 8MHz resonator attached, and drives three LEDs. These LEDs are used to indicate: LED1: An error has occurred, most likely in decoding a message from RS232, RS485 or the uplink RPC stack. LED2: Each time a message is correctly received this LED changes state. So when the unit is controlled by a computer or some RPC stack master the LED should flash continuously. LED3: This LED changes state each time all of the configurable equations have been executed. PL3/IC3 connect 8 user-defined analogue inputs to the ADC (Analogue to Digital Converter) in the processor. PL4 and the attached resistors are the 8 bi-directional serial links for connecting adjacent units together in a system that can pass train speed settings from throttle to throttle. IC6 (optional) extends the space available for the user-defined equations. The processor has a 512 byte EEPROM. SW1 defines the RS485 address that the QTU responds to. PL1 is the upstream (towards the computer, RPI or PTP) RPC stack interface. PL2 is the downstream RPC stack interface. It can have any other RPC modules attached, including other QTUs. ISP1 allows the microcontroller to be reprogrammed without removing it from the board. Q1 functions a a brown-out detector and resets the controller when power is applied, or if the power level drops unexpectedly. IC7, IC8 and SK1-SK3 are the RS-485 interface. Three connectors are provided to allow very flexible interconnect. Typically you might have a chain of QTU units around a layout, but the third socket allows some to be mounted as spurs off the main chain. IC9 and SK7 are the optional RS232 connection. Typically only one of these is required in a computer controlled layout, other QTUs being connected over RS485 and more QTUs and/or other RPC modules being connected to the downstream stack of the others. Note that AC power is connected to two of the three RS485 connectors. This is NOT intended to imply that main track power is distributed over RJ45 connectors, this is not the case. This power feed is only intended to support single wire control panels. Page 2: Input/Output PL5, 6, 7, 8 and IC19, 20, 21, 22 are in essense an SRI4 module that provide 32 general purpose input lines. These are polled every millisecond by the microcontroller and are used as inputs to the user-defined equations. IC11 to IC16 are three quarters of an SRO4-like circuit. These 24 lines are general purpose outputs. The other 8 lines are on page 3. Two or four of the 24 lines may be used to drive one or two Tortoise turnout motors directly. Q2 and Q3 (and the second copy of this circuit) turn the open-collector outputs into full push-pull outputs to provide full reversing of the drive motor, but are current limited (by the 180K resistor and the transistor gain). Page 3: Power supply and throttle output. CN1 is the main 15V AC power feed. From this the following supplies are derived: BR1 generates a rectified but unsmoothed DC which is the main loco power. D1, C1 produce the main smoothed power, around 22V DC. IC23 regulates this down to 12V for the relays and off-board DPR modules. If you prefer 24V relays then omit IC23 and link its pin1 to pin3. IC24 produces a regulated 5V for the logic chips and attached RPC stack modules. C22, C23, D2, D3 are a voltage doubler, then Q6 regulates this down to V+ plus around 3V, to feed the op-amps. The idea is to feed the op-amps a few volts higher than the peak output voltage. This is because the op-amp output is limited to its supply minus 2V. Also the output transistor further reduces the possible track voltage by its Vbe (another 1.2V). D4, C24 produce a suitable +ve bias for any attached FTC modules. D5, D6, C25, C26 (another voltage doubler) produce a -ve bias for FTC modules. D7, D8 produce a separate rectified AC that is fed into a comparator in the microcontroller, and the output is used as a zero-crossing interrupt to synchronise the output circuits. PL13 makes all the above power supplies available to off-board modules. IC17, 18 are the last 8 bits of the on-board SRO4. 4 bits are used as revering relay drivers, and the other 4 bits drive user-accessible relays. In other words the QTU contains half a DPR module. IC10 is a four-channel DAC (Digital to Analogue Converter). The four output voltages drive four identical throttle output circuits: IC25a and TR100 are a simple throttle output circuit. The output voltage (after R105) is fed back to the op-amp which maintains the output voltage to the level set by the DAC. R105 and Q101 form a conventional current limiter. R105 is chosen to suit the current used by your locos. The unit should suit any scale from Z to G. The attenuated track voltage is fed to IC2 (an analogue multiplexor) so that the micro can sample the track voltage to measure feedback volts. R101 measures the track current. The voltage dropped across R101 is duplicated across R102 by IC25b. The current through R102 is the same as the current through R103. Therefore the voltage across R103 is proportional to the current fed to the track. This allows the micro to monitor current consumption - to both detect current flow and provide foldback protection if a motor stalls. Note that this current detector is before the throttle output transistor and therefore inserts no voltage drop to the track (unlike an FTC module which drops over 3V).