Datex-Ohmeda S/5 monitors
Document No. 8001013-1
8
NTC
to AD
converter
+5Vreference
offset
A1
0 °C => 7K357
15 °C => 3K541
25 °C => 2K253
38 °C => 1K301
45 °C => 984R1
Figure 5
Temperature measurement principle
Serial communication
Serial communication between the module and the Central Unit is done by RS485 type bus whose
buffers get their supply voltage (+5 VDC) from the Central Unit and in the isolation section get the
supply voltage (+5 V) from the isolated power supply.
The communication drivers are controlled by a reset signal such that when the reset is active the
drivers do not transfer data.
Reset is also RS485 type and additionally, there is an auxiliary logic power reset, which keeps the
reset active for about 500 ms despite the state of reset in the module bus. A time constant
determines the power-up reset time. There are components to prevent the module from sending
data during reset. The data transmission rate is 500 kbps.
RS485
Driver
RS485
Driver
Reset
Data
NData
Reset in
NReset in
Send data
Receive data
send/receive
Receive data
Send data
send/receive
Reset
to
mod
u
le
p
ro
s
ess
o
r
to
ce
nt
ra
l u
ni
t
(m
o
d
ul
e bu
s)
Opto isolation
Pa
ti
en
t
Is
o
la
ti
o
n
Figure 6
Serial communication and opto isolation section
Isolated section
There are two opto isolators, one for data and one for the reset signal. Signals are processed on
logical high-low levels even though the output of the opto isolators in the isolation section are
analog signals.
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