U N D E R S TA N D I N G Digital Logic ICs
can be used to directly drive as many
as 10 Standard inputs.
Figure 18 presents the above data
in tabular form, together with similar
data for all other major TTL sub-families. When working within any
one sub-family, note that the most
important figure here is the ‘worst case
fan-out (F-O)’ value. Thus, if you are
(for example) designing a system based
entirely on LS ICs, you can confidently
connect an ordinary output directly to
as many as 20 normal inputs, without
risk of a malfunction due to overloading (if you need to drive more than 20
inputs, you can do so via one or more
high-fan-out buffers, etc.). Note that,
within any given sub-family, all ordinary
inputs are said (in TTL jargon) to have a
fan-in of unity (1), but that in practice
some MSI or LSI ICs (such as counters
and registers, etc.) may have special
inputs (such as Reset or Preset, etc.)
with fan-in values of two or greater.
Sometimes, an engineer may have
to mix TTL sub-families, usually so that
an obsolete IC can be replaced by a
readily-available modern plug-in close-equivalent. In such a case, it is necessary
to relate the fan-out data of one sub-family to that of another, to check that
the mix can be made without causing an
input or output overload. One easy way
of doing this is to simply transpose the
data of Figure 18 into ‘Standard TTL’
fan-in units, as shown in Figure 19, to
gain an approximate idea of the relative
fan values of various sub-families. Thus,
it can be seen at a glance that LS TTL
has only half of the fan-in requirement of
Standard T TL, but also has only half of its
fan-out capability, etc.
An even more useful way of
applying the basic data of Figure 18 is to
convert it into an easily-used form that
relates the fan-in and fan-out data of
each TTL sub-family to all other TTL sub-families, as shown in Figure 20. Here, by
reading across the left-hand columns, it
can (for example) be seen that a normal
LS output can drive up to five Standard
TTL inputs, and that a Standard TTL output can safely drive up to 20 LS inputs.
Thus, if an engineer is faced with
a problem such as that illustrated in
Figure 21 — in which a fault on an old
Standard TTL circuit is traced to a
defective 74XXX-type IC (IC2) which
is used to directly drive four other
Standard TTL inputs — it can be quickly seen that a 74LSXXX plug-in equivalent IC can be safely used to directly
replace the IC2 Standard TTL device
without incurring overload problems.
TTL Basic Usage Rules
It is usually a fairly easy matter to
design logic circuitry using TTL ICs,
providing that a set of TTL basic
usage rules are observed. Assuming
that the matter of fan-in and fan-out
has already been taken care of,
there are four basic usage themes
outstanding, and these will be
described in next month’s installment, under the general headings of
Power Supplies, Input Signals, Unused
Inputs, and Interfacing. NV
NEW! HIDmaker FS for Full Speed FLASH PIC18F4550
Creates complete PC and Peripheral
programs that talk to each other over
USB. Ready to compile and run!
• Large data Reports
• 64,000 bytes/sec per Interface
• Easily creates devices with multiple
Interfaces, even multiple Identities!
• Automatically does MULTITASKING
• Makes standard or special USB HID
NEW! “Developers Guide for USB HID
Peripherals” shows you how to make
devices for special requirements.
Both PC and Peripheral programs
understand your data items (even odd
sized ones), and give you convenient
variables to handle them.
PIC18F Compilers: PICBASIC Pro,
MPASM, C18, Hi-Tech C.
PIC16C Compilers: PICBASIC Pro,
MPASM, Hi-Tech C, CCS C.
PC Compilers: Delphi, C++ Builder,
Visual Basic 6.
HIDmaker FS Combo: Only $599.95
DOWNLOAD the HIDmaker FS Test Drive today!
August 2006 59