INFICON STM-2XM 2-Channel Rate/Thickness Monitor - Manual (Page 84)

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STM-2XM Operating Manual
5.3.1 RS-485 Line Signal
The first bit, a start bit, is a data zero value, with the non-inverting signal line going
to a logic low (<1 volt). The opposite differential signal (inverting RS-485) goes to
a logic high (>2 volts) during the same start bit time.
NOTE: After the start bit, the eight data bits are presented in order from least
significant bit (D0) through most significant bit (D7), and then followed with
a stop bit (logic 1 value).
The values of the ASCII code bits as 1 or 0 are represented as normal logic
(1 is high TTL, 0 is low TTL) on the non-inverting RS-485 line.
For example, the opening protocol character STX (ASCII 0x02) is transmitted on
the non-inverting RS-485 wire START0-0-1-0-0-0-0-0-0-STOP1. This can be
examined on an oscilloscope.
5.3.2 Bit Time
At 115200 baud, for example, a bit time is 8.68 microseconds (  s). The start and
LSB zero values together would hold the line low for 2*8.68  s or 17.36  s. Then
the D1 bit value of one would take the line high for 8.68

s.
The remainder of the frame data bits are a zero value, until the final stop bit one
value returns the line to the Idle / 1 / Marking state. At the end of the master
transmission, the line driver for the master enters a high-impedance, inactive state,
so that the slave can reply over the same two wires by sending back a similarly
encoded frame. This master/slave, command/reply interaction is at the heart of the
protocol.
5.4 Bauds
Sycon Protocol must be 9600 baud.
All STM-2XM on the same network must have the same baud rate.
5.5 Message Format
5.5.1 Sycon Protocol
The general format for sending and receiving commands using Sycon Protocol with
STM-2XM uses a start character, a length, data, and a checksum.
NOTE: Each field in angle brackets (< >) is a byte, and is not optional. Fields in
regular brackets ([ ]) are optional. Ellipses (...) mean one or more of the
previous.