SERIAL COMMUNICATIONS
36
Several types of information may be transmitted or
received by the control. The serial communi-
cations capability allows any combination of count
and preset values to be printed, remotely dis-
played, or sent to a host computer or other periph-
eral device for processing. The characteristics of
the communication are controlled by function
codes.
COMMUNICATION FORMAT
The control uses a 20 milliamp current loop type
of electrical interface for serial communications.
The control has a separate 20 milliamp current
loop for incoming communications and another
loop for outgoing communications.
Since serial communication (either in or out) is
done through only two wires, each character trans-
mitted or received must be generated by a series
of on and off states called bits. Each character has
its own unique code or sequence of bits that
allows the receiving device to understand what
character it is receiving. The character “5”, for
example, has a series of bits which are different
from the series of bits for the character “6”. In fact,
eight individual bits are needed to express a single
character. Seven bits identify the character itself
and the eighth is used for error checking to allow
the receiving device to make sure that the previ-
ous seven are correct when they are received.
This eighth bit is called the parity bit and shows
“even parity” to the receiving device when trans-
mitting data. When the counter receives serial
data, it ignores the parity bit.
There are several different standard rates at which
serial communications occur. Each is a function of
the number of bits transmitted per second. The
term which defines transmission rate is “Baud”
which is understood to mean “bits per second.”
The standard transmission rates the control can
be set up to use are 110 Baud, 300 Baud, and
1200 Baud.
While each character requires eight individual bits
to be uniquely expressed, a few additional bits
must be sent between characters. These are
called “start” and “stop” bits. The “start” bit signi-
fies that this is the beginning of the character and
the next eight bits are the character itself. After the
character is transmitted, either one or two “stop”
bits are sent to indicate that the character has
been completely transmitted. When the control is
operating at 110 Baud, two “stop” bits are sent and
at 300 or 1200 Baud one is sent. Thus, at 300
Baud, for example, each character requires ten
bits to be transmitted: one “start” bit, eight data bits
and one “stop” bit. If information is being communi-
cated at 300 Baud, 30 characters per second are
communicated since a total of ten bits per charac-
ter are required.
The standard set of codes used by the control for
communicating information serially is called the
ASCII character table. ASCII stands for American
Standard Code for Information Interchange. The
control uses ASCII codes for all its communica-
tions.
A typical character transmitted or received is
shown in Figure 30. In this figure, the character is
shown with the “start” bit, seven data bits, the even
parity bit, and one “stop” bit.
SENDING DATA
Data transmission can be initiated by either of two
methods. The first is by connecting the PRINT
REQUEST terminal (terminal #16) to DC Com-
mon. The second is by a special code transmitted
to the control via the serial communications.
Once a transmission has been initiated, the
counter will first transmit the “Carriage Return” and
“Line Feed” characters (described in the following
paragraphs and illustrated in Figure 30) followed
by the numeric information selected for printing.
The “Carriage Return” and “Line Feed” characters
cause the printer to provide spacing between print-
outs.
When the control transmits the actual value or
either preset value through the SERIAL DATA
OUTPUT (SDO) terminals, it sends the characters
“0” through “9” as necessary to express the value.
It transmits the most significant digit (MSD) first.
For example, if the current value of the counter is
1357, the control sends the ASCII code for “0”
since the most significant digit is blank and has a
value of zero, then the code for “1”, then the code
for “3”, then “5”, and finally “7”.
