The DS300 Lap Counter/Timer System
Here it is. Each two lane unit is sold separately. The "master" unit
for lanes 1 and 2 goes for $199. "Slave" units provide additional lane
displays for lanes 3-4, 5-6, and 7-8. The "slave" units sell for $99.
You're looking at $300 worth of lap counters here.

Counters out of the box
You can see a difference between the units here. The lane 1-2 master unit has
controls on top. The lane 3-4 slave unit has a plug on top for interfacing to
the master unit.

View of controls
(click on image for full size)
Controls (from left to right) are:
"go!" - for starting and stopping races
"mode" - for selecting race mode (Time, Laps, Formula 1)
"0000" - for selecting digits when programming
"0-9" - for incrementing digits when programming
"LAPS/TIME" - selects display of lap count, time behind leader depending on mode
"CHRONOS" - selects display of fastest and second fastest lap for each lane

Counters (backside)
Here, you can see the lane 1-2 master unit has (from left to right) a power jack,
"stop-go" box interface, computer serial interface and sensor interface.

A Peek Inside the Box
Here, you can see the inside of a lane 1-2 master unit.

A Peek Inside the Box
Here, you can see the inside of a lane 3-4 slave unit.

Counter installed with Stop&Go Box
An optional "Stop&Go Box" controls track power during the race.

Counters installed with optical sensors
In this installation, a Good Year light bridge illuminates optical sensors in
the track. Custom circuitry was built to interface the optical sensors to
the DS300 lap counter. Unfortunately, the DS300 does not support direct
interfacing to optical sensors. It appears to be designed for a trip track
or optical sensors with external circuitry.

PC Board for Optical Sensor Interface
Here is the optical sensors interface board for the DS300 Lap Counter.
The wires on the right hand side go to phototransistors mounted in the track.
Wires on the left go to a 5-pin DIN connector which plugs into the DS300.

Schematic for Optical Sensor Interface
This is the circuit I used to interface optical sensors to the DS300 Lap Counter.
You could get away with less but this design is rock solid.
The DS300 requires positive polarity on the sense signal.
(Click on image for larger view.)

Layout for Optical Sensor Interface Board
The circuit board design was created with ExpressPCB. Here is the layout file: InterfaceBoardRev1.3a.pcb

The software to view/modify the board can be downloaded for free from: expresspcb
They will build three of the boards for you for $51.

More information on the circuit can be found in the PDF file here:
Optical Interface for AFX lap counter/timer

Pinout for DS300 Input Connector
Signals enter the DS300 through a 5-pin DIN connector. The signals
are shown. The sense signals look for a positive voltage. You can use
a trip-track that shorts the +7.5 volts to the sense signal or, as we did, you
can use a circuit which provides positive sourcing for the signal.

"PC Lap Counter" software connected to counters
There are at least three or four lap counting applications for the PC
which communicate to the DS300 through its serial port:
SlotTrak V8.15+,
WinSlot from Cric Crac,
PC Lap Counter from PcLapCounter.com
RaceManager
and
RallySlot (Spanish).

Counters in action in F1 mode
(Click play to watch video)
The "Formula 1" mode is like the "Laps" mode except that, instead
of displaying the absolute lap count for each racer, it displays the
lap count for the race leader and the lap difference for the remaining
racers (e.g. "-1", "-2", "-3", etc).
In all modes, it flashes the lap time momentarily when you cross the finish
line. After about a second, it reverts back to the regular display (laps or
total time).