Max Machinery 120 FLOW RATE & TOTAL INDICATOR - Manual (Page 20)

Page 20 120-200-350 © 1993 (Rev 5/97) Max Machinery, Inc.
RS-232-C TO RS-485 CONVERTER
If you have a computer which has a RS-232-C port and no RS-485 port, it is possible to communicate
with a Max 120 Indicator using a RS-232-C to RS-485 converter. If you have tried to interface two
pieces of equipment from different manufacturers, you know that the serial communications can be a
hassle because there are so many ways to do it while still meeting (or almost meeting) the standard.
RS-232-C communications can be done with as few as three wires (lines), or more if handshaking is
used. Our objective here is to show you how this version of the RS-232-C and RS-485 standards works.
A RS-232-C to RS-485 converter is available from Max (P/N 1-490-38050 ) which has a 9-pin RS-232-
C connector on one end and a terminal block on the other end. In the transmit mode, a Maxim 232
integrated circuit converts RS-232-C to TTL logic levels and a SN 75176 differential bus transceiver
converts the TTL logic level signals to balanced transmission line outputs for RS-485. The reverse
happens in the receive mode.
The RTS signal is used by the host computer to select transmit vs receive mode.
To send a command: RTS = 1 (string is sent out serial port).
To receive a response: RTS = 0 (string is received in serial port).
Specifications
Power Input: 120 VAC +/- 15%
RS-485: Conforms to the EIA RS-485 standard. RS-485 is a differential, half duplex, multi-drop
protocol requiring a total of two wires. All voltage specifications are for terminal 1 (COMM+) with
respect to terminal 2 (COMM-). Communication direction is controlled by the RS-232 RTS signal.
Input (RTS low):
1 condition (mark): -200mV max.
0 condition (space): 200 mV min.
Impedance: 12 K ohms
Hysteresis: 50 mV typ.
Output (RTS high):
1 condition (mark): -1.5 V max.
0 condition (space): 1.5 V min.
(into 54 ohm load.)
Maximum communication rate: 19200 baud
RS-232-C TO RS-485 CONVERTER