11.4. The dumper program

When we run our program, we are going to want to inspect the data that is being read out from the hardware. To do this, we will use the dumper program. But first, we need to understand a little bit about a ring buffer (a.k.a. a "ring").

11.4.1. A very brief introduction to ring buffers

A ring buffer is a fundamental component of the system NSCLDAQ uses to pass data from one process to another and is where the readout program sends its data. Other processes can then read the data from that ring buffer. From the vantage point of the ring buffer, the program filling it with data is its producer and the programs that read from it are its consumers. There is only allowed to be a single producer per ring buffer while there may be many consumers.

Ring buffers are local to a specific computer but are accessible over the network. Each ring buffer is identified by a user-defined name and the hostname of the computer it is located on. When processes want to attach to a ring to either produce or consume its data, they must specify the name of the ring via a universal resource identifier (URI). The URI specifies the protocol (proto://), the hostname (host), and the name of the ring (name) as a single string: proto://host/name. When a user attaches to a ring on another computer, a service running in the background called RingMaster sets up the connection that will stream the data across the network for you. In this way, the nework is basically transparent.

11.4.2. Starting up the dumper program

The program provided by NSCLDAQ for consuming data from a ring buffer and printing it to a terminal is called dumper. It is probably the most fundamental diagnostic provided by NSCLDAQ and is incredibly useful in understanding the integrity of a Readout program. We will use it inspect the data outputted by our Readout program.

To start the dumper program, log onto any computer on the same network as the one physically connected to the VME crate in which your CAEN V785 has been installed. Typically this is the same machine, but it doesn't have to be. To launch the program, you have to specify the hostname of the computer running the Readout program and also the name of the ringbuffer to connect to. The default values for these are "localhost" and your username. Assuming the Readout program is running locally (i.e. hostname=localhost) and your user name is "user0", you would type either:

spdaqxx> dumper --source=tcp://localhost/user0

or equivalently,

spdaqxx> dumper

The dumper terminal session will probably start spewing stuff at to the console once the run starts at a rate that is unreadable. It is generally useful then to limit the number of ring items processed by the dumper by passing the --count option. The value provided to this at launch determines the number of items to process before exiting.

You should now attach this to the ring buffer that will contain your Readout program's output. Its name will default to your username. For illustration purposes, we once again assume your username is "user0". Here is how to tell the dumper program to dump the first 10 events that pass through it.

spdaqxx> dumper --source=tcp://localhost/user0 --count=10

If this failed with some output like this:

spdaqxx> dumper --source=tcp://localhost/user0 --count=10
Failed to open data source tcp://localhost/user0
No such file or directory

it just means that the ring buffer has not been created yet. You should create it and then start your dumper program again. Here is how you do that:

spdaqxx> ringbuffer create user0
spdaqxx> dumper --source=tcp://localhost/user0 --count=10