Introduction; Electrical Connection; Protocol; Preparation - ABB 4620 User Manual

1 INTRODUCTION

This manual must be read in conjunction with the appropriate
Operating Instructions, depending on the instrument type:
• Models 4620, 4621, 4625 & 4626
Conductivity Transmitters
• Models 4623 & 4628 Conductivity
Transmitters to meet USP 23
• Models 4630 & 4635
pH Monitors
• Models 4640 & 4645
Dissolved Oxygen Monitors
• Models 4670 & 4675
Turbidity Monitors
For repeatable and reliable serial communication to take place
between a master (host computer) and slaves (instruments) it
is essential that the two conditions detailed in this section are met.

1.1 Electrical Connection

A standard method of electrical connection is used between
the master and the slaves, with defined voltage levels and
characteristics. The transmitter and receiver integrated
circuits within the 4600 meet the requirements of the EIA
(Electronic Industries Association, American) RS485 and
RS422 Serial Interface Standards.
The RS422/485 communication standard is used with the
following logic levels:
a) for logic '1' (MARK condition or IDLE state) the 'A'
terminal of the transmitter is negative (0V) with
respect to the 'B' terminal (+5V)
b) for logic '0' (SPACE condition or ACTIVE state) the 'A'
terminal of the transmitter is positive (+5V) with
respect to the 'B' terminal (0V).
Note. The 'A' terminal is Tx + or Rx + and the 'B'
terminal is Tx – or Rx –.

1.2 Protocol

A standard language or protocol must be used in both the
master and the slaves for messages (commands and data) to
be interpreted and acted upon. To achieve this second
condition, Modbus Protocol is utilized on the 4600 Series
Monitor using the Remote Terminal Unit (RTU) mode only.
Two methods of message error checking are used. Parity
checking is used, if selected, to detect transmission errors in
individual characters.
Parity is used for simple error checking. The parity bit is a one-
bit code which is transmitted in addition to the ASCII character.
It can detect only one error per character, since two errors may
cancel out. Parity is calculated by finding the sum of logic '1's
in the character and either:
a) setting the parity bit to logic '1' if the sum is odd, or logic
'0' if the sum is even, when using even parity.
or
b) setting the parity bit to logic '0' if the sum is odd, or logic
'1' if the sum is even, when using odd parity.
Cyclic Redundancy Checking (CRC-16) is used to detect errors
in the Master messages and Slave responses. This therefore
detects errors in the complete message sent and also the replies.
2

2 PREPARATION

Preparation is as given in the Operating Instructions
– see Section 1, with additions as detailed in this section.
– IM/4600–CON

2.1 Company Standard Settings

Only those parameters detailed on the customer order are
– IM/4600–USP
programmed at the factory. If any parameters are unsuitable
for the application they can be reprogrammed – see Section 7
– IM/4630–PH
of the Operating Instructions . Serial data programming details
are given in Section 7 of this manual.
– IM/4600–DO
Standard parameter settings for the serial data programme
– IM/4670
are as follows:
Instrument Identity
Parity
Transmission Rate
01
none
9600 baud.