Physics and Engineering of Radiation Detection

724 Chapter 13. Data Acquisition Systems

and readout through a computer. These ports are necessary due to unavailability of
digital data bus in NIM crates.
As mentioned earlier, NIM modules are well suited for small scale systems that
do not require complicated data handling and control. Most of the NIM modules are
simply plug-n-play devices, that is, one can develop a whole data acquisition system
by simply interconnecting off-the-shelf modules together in the right sequence.

A.3 NIMLogic

Standard NIM modules are designed to deliver and respond to slow-to-medium pos-
itive logic signals and fast negative logic signals. Since fast negative logic signals are
more popular, they are generally said to constitute the standardNIM logic.These
negative levels have been defined in terms of current ranges. However since the 50
W input/output impedances are also a part of the standard, the levels can be ex-
pressed in terms of voltages as well. The current standards are listed in Table 13.2.2.
These fast negative pulses can have rise times as short as 1ns. This makes them
susceptible to reflections and must therefore be properly terminated. The general
practice is to use 50Ω coaxial cables, such as RG-58 or RG-174, terminated with
50Ω.

Table 13.2.2: Standard fast negative NIM logic levels.

Type Input (mA) Output (mA)

Logic 0 -4 to +20 -1 to +1

Logic 1 -12 to -36 -14 to -18

The levels corresponding to the slow-to-medium positive logic signals are listed
in Table 13.2.3.

Table 13.2.3: Standard slow-to-medium positive NIM logic levels.

Type Input (V) Output (V)

Logic 0 +1.5 to -2 +1 to -2

Logic 1 +3 to +12 +4 to +12

The reader would note that these standards are fairly different from other com-
monly used ones, such as TTL and ECL. This is a potential problem since it could
limit the integration capability of NIM modules with systems based on other stan-
dards. To overcome this difficulty TTL-NIM and ECL-NIM translation modules
have been made available. This added versatility has significantly broadened the
scope of applicability of NIM based systems.