Timing hardware


What timing hardware is compatible with SRM?
SRM needs basic timing hardware connected to one or more parallel (printer) port(s). Basic timing hardware is actually one "switch" per lane that detects a passing car and feeds this info to the computer in the form of a "0" or a "1". The switch can be:
- photodiode or a photoresistor (normal light or infra red) <- preferred,
- (Reed) switch,
- dead strip,
- magnetic sensor.
It can be wise to add circuitry to provide more clean pulses and to prevent damage to the parallel port.
From version 2.01 and up, SRM is fully compatible with all these sensors connected to parallel ports. SRM supports multiple ports, so you can use up to 3 parallel ports on one computer to time different tracks with up to 8 lanes per track. Inverted bits/pins are also automatically supported and corrected.

Is all Timing Hardware compatible with SRM?
Only parallel port devices are supported. Serial port, mouse port or USB ports are not supported.
There are also "intelligent timing devices" on the market, which have their own timing hardware built in (DCS, Ninco Pole Position). Sometimes these devices have a (parallel) cable to connect the device to a PC, but these systems use a data stream instead of pulses, which makes the device incompatible with SRM.

Building your own hardware
If you want to use SlotRace Manager (SRM) or any other timing software using the parallel port, you need some hardware to do this. You can easily build your own timing hardware for use on a PC's parallel port using light sensitive sensors, using one of the next ways:

a. Universal setup
pro's: works on al PC's
con's: more complicated & expensive to build (uses IC's and more complex circuitry)

b. Personal setup
pro's: only sensors, wire and a D-connector, so cheap and very easy to build
con's: built specifically for your pc, so it might not work on another pc

This information is about building a simple, personal setup which will work almost every time! To achieve this, you will have to follow the next steps:
1. determine the resistance of your sensors (at light & dark)
2. determine the threshold of your pc's parallel port
3. make corrections if there is a difference between 1 & 2
4. build the hardware

Stuff needed:
- old PC, at least a 80286, with a working parallel (printer) port,
- light sensitive sensors (photodiode or photoresistor sensors, about $2 to $4 a piece),
- some form of light source above the sensors,
- 25 pin D-connector, fitting on a parallel port (few bucks),
- computercable, preferably shielded (few bucks),
- resistor collection with a range of resistors, preferrably 100ohm and 1000ohm (few bucks)
- multimeter (for measuring resistance in ohm),
- an hour of your time :-)

How does the software reads a passing car?
A parallel port has input pins that can be switched (ON or OFF) from outside the computer. The preferred pins to use are numbered 10, 11, 12, 13 and 15. If you need more, you can also use pins 1, 14, 16 and 17. The condition of these pins depend on a connection between a ground pin (numbered 18 to 25, choose one) and the input pin, using a 200ohm resistor:
- „no connection” (='high resistance') results in '0' (off)
- „connection” with a small resistor (='low resistance') results in '1' (on).
On occasion this is just the reverse (so called „inverted pins”), but SRM automatically compensates this.

Caution: DO NOT connect ground pin to a data pin without a resistor, you will destroy your parallel port!


SRM reads the state of these input pins and uses them to trigger the timing. The only thing we need to build is something that detects a passing car and then switches the input pins. For this purpose we use a photoresistor or photodiode, which actually is a resistor that changes under different light conditions.

STEP 1 - determine the resistance of your sensors (at light & dark)
Put the photosensors in a position as they will be built in your track and apply the same light (using a light bridge or an extra light source). Now use the multimeter to measure the resistance of the photo sensor (='light').

Put an object (a car) on top of the sensor and measure the resistance again (='dark'). Apply this routine on all available sensors and write down all 'dark' and all 'light' resistances.

Example: you have three sensors:
Sensor 1: 'dark' = 8.800ohm, 'light' = 900ohm
Sensor 2: 'dark' = 2.000ohm, 'light' = 600ohm
Sensor 3: 'dark' = 12.800ohm, 'light' = 5.600ohm

STEP 2 - determine the threshold of your PC's parallel port
For this purpose, you will need a program called PARPORT.EXE, which is included in the SRM download (found in the download section). Run PARPORT program during this step.

Make some sort of setup which allows you to place a range of resistors between a ground pin and an input pin. Start with using a resistor of 200ohm. Keep your eye on PARPORT and connect/disconnect a few times. If your port is working correctly, you will notice one of the pins switching from '0' to '1' (or vice versa).

Increase the resistance from 200ohm using steps of 500ohm or 1000ohm by adding extra resistors in the chain. Check PARPORT after each increase. Eventually you will reach a point when PARPORT will not switch from '0' to '1' anymore. Now make a note of the total resistance you had on your previous step, the last step PARPORT reacted on with a connection/disconnection. That resistance is your PC's threshold value. If you want, you can fine tune this value, using the same procedure with smaller resistance steps (you may need this, depending on the sensors you will be using).

Example: your PC's threshold is at 3.000ohm.

STEP 3 - make corrections, if necessary
I will explain the corrections by using the results from the examples from photosensors 1, 2 and 3 in step 1. The goal is simple: put the PC's threshold (3000ohm) somewhere between the values for 'light' and 'dark' for each sensor.

Sensor 1
If we look at the 'dark' and 'light' values of sensor 1 in the example, we see that these values are both on a different side of the PC's threshold. 'Dark' is above it, 'light' below' it. This is perfect, because if a car passes it will be dark for a short while, resulting in a short switch from '0' to '1' and back again: the resistance rises and passes the threshold for a moment, then drops again back to the 'light' value. SRM detects the car.
Correction: not needed.

Sensor 2
This sensor has both values 'light' and 'dark' below the PC's threshold. When a car passes, the sensor's resistance will rise from 600 to 2000ohm, but not high enough to pass the threshold. This way, the input pin will not switch from '1' to '0' and the PC will not read the passing car.
Correction: Add an extra resistor of 1.500ohm in a serial manner (before or after the sensor). This way the total resistances rise and 'dark' becomes 3500ohm (=2000+1500) and 'light' becomes 2100ohm (=600+1500).

Sensor 3
For this sensor, both values for 'dark' and 'light' are above the PC's threshold. If a car passes, the sensor will detect this, but the resistance for light is already above the threshold and will only rise more with 'dark'. The threshold will not be passed, there will be no trigger and the PC (and SRM) will not detect the car!
Correction: The total resistance of the photosensor must come down by 5.000ohm. This way the PC's threshold will fall between both values of the photosensor. To lower the value of a resistor in a circuit, you will need to place another resistor parallel to it:

To calculate the value of that resistor, you need this formula to calculate the total resistance of the circuit:

R(total) = 1 / ( (1/R1 ) + (1/R2) ) , with:

R(total) = total resistance
R(1) = resistance of photosensor in either position (light/dark)
R(2) = second, parallel placed resistor.

If you dump this formula in a spreadsheet, you can calculate the value that R(2) needs to be, to get the PC threshold in between the 'light' and 'dark' resistances of the photosensor.

In this case R(2) = 4.800ohm:
'dark' = 1 / ((1/12.800) + (1/4800)) = 3490ohm
'light' = 1 / ((1/5.600) + (1/4800)) = 2585ohm

STEP 4 - build the hardware
In my opinion, 'hardware' is a rather large word for the setup we need to build. Here is a schematic drawing of the setup:

In the most simple case, you have only sensors like sensor 1 in our example. I came across that situation in both of the last setups I built. In that case, (S1), (S2) and (S3) in the schematic are only the photosensors.

In the case you have a sensor like sensor 2 or 3 of the example, you will need to replace (S1), (S2) or (S3) in the schematic with the total setup, so with the serial or parallel placed resistors added!

Place of the timing sensors in your track and the speed of the Photosensor
When you go out shopping for the photosensors, be shure to get sensors that switch fast enough to cross the PC threshold in the very short time a car passes the sensor. In most cases the sensor has a switching time of 25 to 35ms (milliseconds), which is fast enough for most situations. Faster sensors are more costly.

When looking for a place to put the sensors in your track, always consider this:
Do not pick a fast place in your track: sensors might miss a fast car easier than a slower car.
Do not pick a place right after a corner: a car might sway his butt and trigger a sensor in a lane next to him.

More information
Here, with a fellow timing software site, you can also find a lot of info on building timing hardware: click here.

More schematics for slot racing
Thanks to Bruce Woods, we have a couple of nice schematics for you guys:
- Track power wiring diagram
- Controller station wiring diagram
- Start lights and sensor wiring diagram
- Pulse conditioning schematic