I appreciate Mr. Karnes' academic approach to the solution of the problem, but my understanding is that "nickfromny" needs some simpler, real world hints to get rid of an annoying problem which is actually quite common when dealing with cables connecting buildings and devices, rather than performing investigations which are out of reach for most, both because of the skills required to use sophisticated tools like the aforementioned (oscilloscopes, reflectometers, insulation testers), but also for the cost of this gear, that can easily be higher than the cost of the whole timing installation.
Most people I know dealing with time-keeping have just some very basic knowledge of electrical wiring and have access to very simple tools like low-cost multimeters. Assuming anything beyond this may be pure dream.
First I would like to try to make some points more clear.
Buffer
Every timing device on sale has some kind of buffer.
All pulses from photocells, starting gates, etc. are recorded and 'queued' for late processing. As you may imagine, not only simultaneous pulses coming from more channels but also rapid sequences of impulses on one input channel need to be queued in a buffer (stored in memory and then processed) to accomodate the different latencies of the printer, the serial line, the display etc. and all the peripherals that may be busy or simply not ready to perform the job.
The old Alge Tdc4000 had a 18-positions buffer on the finish channel, no buffer on the start and on the intermediate channels. The start had an adjustable delay that made it impossible to record quick sequences of pulses. The intermediate channel had no buffer (i.e. too tight pulses were simply lost).
Nowadays thanks to the memory availability, all timers feature wide acquisition buffers.
ALL the devices you listed have input buffers.
Optocouplers are not buffers
Optical isolation (I'm trying to simplify as much as possible) is a means to physically interrupt the continuity of wires, preservating the logical state (typically open or short circuit). To make it more easy to understand, think about a pair of wires terminated by a simple switch. You may open or close the switch and the corresponding logical state will be OPEN or CLOSED. Now add and optocoupler. The device will carry the state of the circuit, without being physically connected (light is used to carry the information).
Different voltages may be present on the other side, but the device will only pass through the logical state of the wiring.
PLEASE be aware that one of the dangerous side effects of optocoupling is that hazardous voltages may be present between the input and output of the optocouplers.
In addition to optically coupling, some extra features may be present in these devices, like low pass filters (a pulse lasting some milliseconds may correspond to some tenth of second of output pulse. This is often known as 'delay' even if it is not a true delay, just a longer 'status-hold' on output). The device maker should be able to give you all details.
You may isolate the START wires with tools like the TAG HL-553 or the BlinkyBox,
you may isolate the serial data connection with tools like the Moxa TCC-82.
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Now you need to solve your issue.
I have some suspect about the 'crazy' behavior of your START input as coming from an AC 50 Hz power leak or poor/unsafe grounding. This may also have slightly damaged your timer's inputs.
Make sure that if you use a generator, this SHOULD be grounded following the builder's safety instructions.
Let's go inside the problem. May I suggest a 'divide et impera' approach.
All you need is your timing equipment (everything, even the PC, the headsets and the score board, not just the timer), and an multimeter.
You may go el-cheapo and have one for 30 bucks. A good investment may be a professional Fluke (my favourite is the 179) or a Tektronix one. Hope admin doesn't ban me for advertising.
Unfortunately, it often happens on the hills that older power installations were unsafe, lacking proper grounding. Have an experienced technician check the electrical safety of the hut (sockets, ground, etc.) BEFORE investigating yourself.
If the hut is metal-based, ask to check that it is properly grounded.
Only then you can do some simple tests on your equipment.
Be aware of this fact: since you are dealing with cables going inside & outside the building, the category of the installation is Cat III and Cat IV. This means that in order to work on the wires, you may need special skills, training and wear appropriate protection (shoes, gloves).
Lay down your equipment. DO NOT inter-connect anything.
Just plug the power adapter to the timer, the PC and the score board.
Now check, with your multimeter:
a) The impedance (OHM) of the wires going to the start.
Open and close. Ideally should be infinite (0L) when open, and from 30 to 1000 OHM when closed. I assume the wires are longer than 1000ft;
b) Now check if there are some hidden differences of potential (voltage) between each wire and the timing hut ground (windows and walls if the hut is steel or alu built);
c) Check the voltage each wire and the electrical ground of the power sockets;
d) Check all possible voltages present between the wires going to the start and the chassis of the timer, of your PC, the shell of the serial adapter, in a word EVERYTHING which is not plastic.
Now wear the headsets, and connect them to the same cable used for the start gate.
DO NOT CONNECT the timer. Just the headsets. You should be able to talk to someone at the start without any noise.
Now connect the timer. You may hear some very light noise. If the noise is HIGH, as high to make the start unhearable, AC is passing through and thus needs to be stopped.
1) Unplug the power adapter. The noise disappears? => The adapter (which should be built with double isolation) is faulty and HAS TO BE REPLACED.
2) Connect the scoreboard. If possible check also if there is some voltage between the scoreboard's chassis and the wires going at the start.
If high noise is heard, the board may have some power leak;
3) Connect the PC's serial line (and the printer, etc.).
(Often 'switching' power supplies for the laptops and printers are a source of problems).
In one of these steps, if:
-you hear high noise in the headset,
-the start line gives you thousands of pulses,
-you find some anomalous voltage between wires and tools,
then you have identified your problem and may ask for help in a restricted domain of possible issues.