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Digital Electronics
5.1.2.3 ECL Subfamilies
The first monolithic emitter coupled logic family was introduced by ON Semiconductor, formerly a
division of Motorola, with the MECL-I series of devices in 1962, with the MECL-II series following
it up in 1966. Both these logic families have become obsolete. Currently, popular subfamilies of ECL
logic include MECL-III (also called the MC 1600 series), the MECL-10K series, the MECL-10H series
and the MECL-10E series (ECLinPS and ECLinPSLite). The MECL-10K series further divided into
the 10 100-series and 10 200-series devices.
5.2 Characteristic Parameters
In this section, we will briefly describe the parameters used to characterize different logic families.
Some of these characteristic parameters, as we will see in the paragraphs to follow, are also used to
compare different logic families.
• HIGH-level input current, I IH . This is the current flowing into (taken as positive) or out of (taken
as negative) an input when a HIGH-level input voltage equal to the minimum HIGH-level output
voltage specified for the family is applied. In the case of bipolar logic families such as TTL, the
circuit design is such that this current flows into the input pin and is therefore specified as positive.
In the case of CMOS logic families, it could be either positive or negative, and only an absolute
value is specified in this case.
• LOW-level input current, I IL . The LOW-level input current is the maximum current flowing into
(taken as positive) or out of (taken as negative) the input of a logic function when the voltage
applied at the input equals the maximum LOW-level output voltage specified for the family. In the
case of bipolar logic families such as TTL, the circuit design is such that this current flows out of
the input pin and is therefore specified as negative. In the case of CMOS logic families, it could be
either positive or negative. In this case, only an absolute value is specified.
HIGH-level and LOW-level input current or loading are also sometimes defined in terms of unit load
(UL). For devices of the TTL family, 1 UL (HIGH) = 40 A and 1 UL (LOW) = 1.6 mA.
• HIGH-level output current, I OH . This is the maximum current flowing out of an output when
the input conditions are such that the output is in the logic HIGH state. It is normally shown as
a negative number. It tells about the current sourcing capability of the output. The magnitude of
I OH determines the number of inputs the logic function can drive when its output is in the logic
HIGH state. For example, for the standard TTL family, the minimum guaranteed I OH is −400 A,
which can drive 10 standard TTL inputs with each requiring 40 A in the HIGH state, as shown in
Fig. 5.2(a).
• LOW-level output current, I OL . This is the maximum current flowing into the output pin of a logic
function when the input conditions are such that the output is in the logic LOW state. It tells about
the current sinking capability of the output. The magnitude of I OL determines the number of inputs
the logic function can drive when its output is in the logic LOW state. For example, for the standard
TTL family, the minimum guaranteed I OL is 16 mA, which can drive 10 standard TTL inputs with
each requiring 1.6 mA in the LOW state, as shown in Fig. 5.2(b).
• HIGH-level off-state (high-impedance state) output current, I OZH . This is the current flowing
into an output of a tristate logic function with the ENABLE input chosen so as to establish a
high-impedance state and a logic HIGH voltage level applied at the output. The input conditions are
chosen so as to produce logic LOW if the device is enabled.
Digital Electronics
5.1.2.3 ECL Subfamilies
The first monolithic emitter coupled logic family was introduced by ON Semiconductor, formerly a
division of Motorola, with the MECL-I series of devices in 1962, with the MECL-II series following
it up in 1966. Both these logic families have become obsolete. Currently, popular subfamilies of ECL
logic include MECL-III (also called the MC 1600 series), the MECL-10K series, the MECL-10H series
and the MECL-10E series (ECLinPS and ECLinPSLite). The MECL-10K series further divided into
the 10 100-series and 10 200-series devices.
5.2 Characteristic Parameters
In this section, we will briefly describe the parameters used to characterize different logic families.
Some of these characteristic parameters, as we will see in the paragraphs to follow, are also used to
compare different logic families.
• HIGH-level input current, I IH . This is the current flowing into (taken as positive) or out of (taken
as negative) an input when a HIGH-level input voltage equal to the minimum HIGH-level output
voltage specified for the family is applied. In the case of bipolar logic families such as TTL, the
circuit design is such that this current flows into the input pin and is therefore specified as positive.
In the case of CMOS logic families, it could be either positive or negative, and only an absolute
value is specified in this case.
• LOW-level input current, I IL . The LOW-level input current is the maximum current flowing into
(taken as positive) or out of (taken as negative) the input of a logic function when the voltage
applied at the input equals the maximum LOW-level output voltage specified for the family. In the
case of bipolar logic families such as TTL, the circuit design is such that this current flows out of
the input pin and is therefore specified as negative. In the case of CMOS logic families, it could be
either positive or negative. In this case, only an absolute value is specified.
HIGH-level and LOW-level input current or loading are also sometimes defined in terms of unit load
(UL). For devices of the TTL family, 1 UL (HIGH) = 40 A and 1 UL (LOW) = 1.6 mA.
• HIGH-level output current, I OH . This is the maximum current flowing out of an output when
the input conditions are such that the output is in the logic HIGH state. It is normally shown as
a negative number. It tells about the current sourcing capability of the output. The magnitude of
I OH determines the number of inputs the logic function can drive when its output is in the logic
HIGH state. For example, for the standard TTL family, the minimum guaranteed I OH is −400 A,
which can drive 10 standard TTL inputs with each requiring 40 A in the HIGH state, as shown in
Fig. 5.2(a).
• LOW-level output current, I OL . This is the maximum current flowing into the output pin of a logic
function when the input conditions are such that the output is in the logic LOW state. It tells about
the current sinking capability of the output. The magnitude of I OL determines the number of inputs
the logic function can drive when its output is in the logic LOW state. For example, for the standard
TTL family, the minimum guaranteed I OL is 16 mA, which can drive 10 standard TTL inputs with
each requiring 1.6 mA in the LOW state, as shown in Fig. 5.2(b).
• HIGH-level off-state (high-impedance state) output current, I OZH . This is the current flowing
into an output of a tristate logic function with the ENABLE input chosen so as to establish a
high-impedance state and a logic HIGH voltage level applied at the output. The input conditions are
chosen so as to produce logic LOW if the device is enabled.