Emerson PNC001-A Series User Manual

PACSystems™ RX3i and RSTi-EP CPU Reference Manual Section 6
GFK-2222AK October 2019
Serial I/O, SNP & RTU Protocols 164
Note: Data addresses are 0-based. This means you will need to subtract 1 from the actual address
when specifying it in the RTU message. For message (01) READ OUTPUT TABLE Query, used in
the example above, you would specify a starting data address in the Starting Point No. field.
To specify %Q0001 as the starting address, you would place the address %Q0000 in this field.
Also, the value placed in the Number of Points field determines how many %Q bits are read,
starting with address %Q0001. For example:
▪ Starting Point No. field = %Q0007, so the starting address is %Q0008.
▪ Number of Points field = 16 (0010h), so addresses %Q0008 through %Q0023 will be
read.
6.3.1.6.1 Error Check Field
The Error Check field is two bytes in length and contains a cyclic redundancy check (CRC-
16) code. Its value is a function of the contents of the station Address, Function code, and
Information field. The details of generating the CRC-16 code are described in Cyclic
Redundancy Check (CRC). Note that the Information field is variable in length. To properly
generate the CRC-16 code, the length of frame must be determined. To calculate the
length of a frame for each of the defined function codes, see Calculating the Length of
Frame.
6.3.1.6.2 Message Length
Message length varies with the type of message and amount of data to be sent.
Information for determining message length for individual messages is found in RTU
Message Descriptions.
6.3.1.6.3 Character Format
A message is sent as a series of characters. Each byte in a message is transmitted as a
character. The illustration below shows the character format. A character consists of a
start bit (0), eight data bits, an optional parity bit, and one stop bit (1). Between
characters the line is held in the 1 state.
MSB
Data Bits
LSB
10
9
8
7
6
5
4
3
2
1
0
Stop
Parity
(optional)
Start
6.3.1.6.4 Message Termination
Each station monitors the time between characters. When a period of three character
times elapses without the reception of a character, the end of a message is assumed. The
reception of the next character is assumed to be the beginning of a new message. The
end of a frame occurs when the first of the following two events occurs:
1. The number of characters received for the frame is equal to the calculated length of
the frame.
2. A length of 4 character times elapses without the reception of a character.

Emerson PNC001-A Series Specifications

General

Brand	Emerson
Model	PNC001-A Series
Category	Controller
Language	English