Max Machinery 120 FLOW RATE & TOTAL INDICATOR - Manual (Page 18)

Page 18 120-200-350 © 1993 (Rev 5/97) Max Machinery, Inc.
RS-485 SERIAL COMMUNICATIONS
The Max 120 Indicator is equipped with an RS-485 serial communication port for the purpose of
allowing a computer to:
1. Issue control commands such as reset.
2. Query run mode data such as count, rate, setpoints, etc.
3. Load setpoints.
4. Query and program all program mode sub menus except numbers 13, 15, 42, 43 & 44.
Communicating with the Max 120 Indicator via RS-485 2-Wire Bus
The Max 120 Indicator is equipped to communicate with a host computer via a 2-wire, differential,
half duplex, multi-drop RS-485 bus.
1. All devices (like MAX 120) on the bus wait in the receive mode.
2. The host computer will, as needed, send a command to a specific device on the bus. The
commands are several characters long, length being determined by how much data must be
sent as part of the command.
The second and third characters in a command are unit ID number.
3. When a device recognizes its ID number, it interprets the command, waits a 0 to 500
millisecond time interval which commences with receipt of the last command character’s stop
bit and transmits a response. Then the device goes back to the receive mode and waits.
4. The host computer transmits a command as required and then switches to the receive mode to
await the response from the device to which the command was transmitted.
Communicating with the Max 120 Indicator via RS-232-C Bus
RS-232-C equipped computers can be interfaced to the 120 by using a communications converter. See
page 15.
Programming Examples
Starting on page 24 are examples of the code necessary to communicate with the MAX 120 over the
RS-485 port. There is a version for Mircosoft Quick Basic and a version for HP Rocky Mountain
Basic. These programs are just one of many ways to write a driver to communicate with the MAX
120. These programs are intended as examples and are a guide, but are not intended to be complete
communications package.
RS-485 SERIAL COMMUNICATIONS
RS-485 LINE
RS-485
DEVICE
MAX 120 HOST PC
+