Midas
®
Gas Detector
A-2
A Modbus/TCP Interface
The Midas
®
gas detector can report concentration
information in a variety of formats including relay
contact closure, an analog 4-20 mA loop, Ethernet/
IP/HTML web pages and via the Modbus/TCP
networking protocol. This section defines the format
of data in Modbus/TCP registers. More information
about Modbus/TCP can be obtained from www.
modbus.org. More information on the Midas
®
web
pages is contained in Section 14.
The Midas
®
is a Modbus/TCP “server” as defined
in the MODBUS Application Protocol Specification
V1.1a. It supports command 03 (“read holding
registers”) for registers 40001 to 40018 as listed in
Table 1.
This section describes Midas
®
software V1.08;
previous software release levels support a subset of
these registers.
The Midas
®
reports floating-point concentration
numbers in little-endian or Intel format. This is scaled
in units of ppm for gasses which have display units of
ppm or ppb. For other display units the concentration
is scaled directly in display units. This applies to
registers FCN, A1T and A2T.
A.1 Reading Status from the Midas
®
The Midas
®
fills 18 Modbus/TCP holding registers
as listed in Table 1. The STTS register is a concise
summary of Midas
®
status. The other registers
provide more complete information.
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