KROHNE MFC 010 C Converter - Manual (Page 21)

MFC010 Interface Manual 19
5. Modbus Protocol Interface
The interface to the MFC010 is implemented in the Modbus RTU communications protocol, and i s done
so in accordance with the specification and requirements of the “Modbus Protocol Reference Guide” (PI-
MBUS-300 R ev J ). The phy sical el ectrical par ameters of t he Modbus s pecification ar e def ined by t he
EIA/TIA-485 (RS485) standard and t he “ Modbus ov er S erial Li ne - Specification and I mplementation
Guide V1.0” interface definition.
In a serial communications system such as the Modbus protocol, data is transmitted as a series of voltage
levels along the connecting data wires. A “bit”, or binary digit, value is determined by the logical level
(high or low) of the connecting interface over a set time period. The time period for each bit is determined
by the transmission speed, known as the baud rate. For a baud r ate of 9600, the bit period is 1/9600 =
104.2 microseconds. T he MFC010 supports Baud rates of 1200, 2400, 4800, 9600, 19200, 38400 and
57600 baud (see the Baud rate setting in Holding Register No. 1005). The higher transmission speeds
require careful attention to the cable installation in order to function reliably and error free (see Section 3.2
on page 7 for installation details).
5.1 Character Transmission Format
Data is transmitted in sets of 8 bit data blocks, known as “Bytes” or “Characters”. Each character i s
preceded and followed by framing bits that permit the correct detection of the transmitted character. The
first “Bit” transmitted will be the “Start” bit, this permits the receiving device to detect that a character is
being transmitted. The “Start” bit is then followed by the 8-bit data byte. A “Parity” bit may then follow the
8-bit character. T his “Parity” bit is optional (see the Transmission format setting in Holding Register No.
1004), it allows the system to validate the contents of the 8-bit data byte to ensure that no er rors have
occurred during transmission. Following the “Parity” bit is the “Stop” bit that indicates the end of the
transmitted character to the receiving device. If no parity bit is used, two stop bits must be implemented,
this ensures a consistent character length of 11 bits is maintained.
The 8 dat a bits are annotated from bit 0 ( the least significant bit, LSB) to bit 7 ( the most significant bit,
MSB). The character is transmitted “MSB first”, i.e. the first bit after the start bit, is bit 7 of the data byte.
See appendix B for more details on binary coding.