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My Rockwave VS1 Seismometer  came with a standard 5 m cable, incorporating a  RS232  female serial port plug and a 4 mm banana plug. the 4 mm banana plug connects  into the Rockwave GPS Time module .

It is a good idea to have your computer  clock  synchronise to a accurate timing source, there are many utilities on the internet which can perform this task.  One of my favourite timing  software is called NMEATIME utility. Using the network time settings you can connect to a Internet time server which then synchronises your computers time clock, the software also allows you to use a GPS module  for its time source

If you are part of an  array network you could use the more accurate timing solution by using Rockwaves timing module and Helistamp 6. if you have your computer within the 5 m cable distance.  You should have no problem using the GPS time module with the standard 5 m cable if you have your computer and seismic instrument at very long distance    You may need to make up a new cable, with the 4 mm plug at the end of the female RS232 plug which connects to your computer serial port.  this will make it easy to connect the GPS module to your computer USB port and convenient to plug in the 4 mm plug that will send the one pulse per second down to the male  RS232 (pin6 DSR,)

I have found  Cat5 is adequate to use for this new cable at a distance up to 45 metres which will have female and male RS232 serial plugs and a new 4 mm banana plug for sending the one pulse per second to the instrument RS232 port, using this setup you do not use any converters. You must have  a RS232 port on your computer Com 1.  
4mm Banana Plug
new:-1PPS GPS Plug
Cat 5 cable
Male And female RS 232 plug fittings
VS1 RS232 Female Plug 
DB9 view looking into
the male connector
DB9 view looking into
the Female connector
Plug into the VS1
Plug into a RS 232 port on your computer
Please note  the blue cable needs to connect to a 4 mm banana plug
To make this cable.  You will need  (1) 4 mm banana plug, (1) Male RS232  plug  (1) female  RS232 plug ( cat5 cable maximum length  45 metres)

Please refer to the rockwave GPS module operational guide for more information on how the one pulse per second works in conjunction with the VS1 seismic instrument and Helistamp 6

Please note

I advise, if you are ordering the VS1 instrument from RockWave let them know your cable length  which should eliminate making up a new cable
With the GPS module connected and running on your computer. Using  NMEAtime time utility this is setup to use the GPS time to synchronisation the computer ,data is coming from the  virtual  RS232 port using (com 2).  Also running is  AmaSeis earthquake recording software  getting data from the VS1 RS232 port (com 1)  with Helistamp 6  running .  It needs data from (com 1) and (com 2).

To solve this problem you can use  a piece of software called Serial Splitter this utility can create  shared ports.

With Helistamp 6 running the programme creates a log file with the timestamps seconds since midnight (SSM) and velocity from the VS1,
if I pick up an event I can check the timestamp seconds since midnight against the velocity and compare it with another station using the same system .

One pulse per second dropouts are not recorded.

To convert the SSM I use a small programme that converts the seconds since midnight into UTC time .

An example of the  log  file from Helistamp 6  2013-12-15

49159.512 - 173

UTC time converter
Weather- above is using serial splitter programme with our earthquake applications