Appendix B: Master-Slave Protocol
VT400 Technical Manual, Rev. A5 67 Doc # UM-VT400-EN
Appendix B: Master-Slave Protocol
Appendix Contents
I. INTRODUCTION .................................................................................. 67
II. BAUD RATE AND MESSAGE FORMAT ................................................... 67
III. DATA BLOCK FORMAT....................................................................... 68
IV. COMMUNICATION PROCESS OVERVIEW ............................................ 68
V. MASTER-SLAVE COMMANDS................................................................ 69
i. Introduction
This appendix defines the communication between a host computer (the master) and a
maximum of 5 weighing terminals (the slaves). The host computer may be any
processor connected to an asynchronous serial port: for example, a personal
computer or a mini computer. Each slave in the network possesses a unique address,
in ASCII characters A to Z. The Master-Slave Commands enable remote monitoring
and/or operation of the weighing terminals by the host processor.
The Master always controls the communication link although the Master itself may at
the same time be a slave to a higher level network. The master polls the slave
machines and they respond within a specific time period.
The protocol is designed as a low-cost long-distance communication link, to support
sporadic data exchanges in industrial environments, for network multi-drop
applications. Network connections require twisted-pair shielded cables and support a
20mA current loop standard and the RS485 standard.
ii. Baud Rate and Message Format
600-9600 baud,
1 start bit, 7 data bits, 1 even parity bit, and 1 stop bit.
6 data bits are used for the communication; The 7th data bit serves as the start-of-
message delimiter in the address byte.
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