INM 5500 Rev 11
72
10 .3 .2 Hazardous-area interfacing
Displays/terminals: For details of interfacing with MTL640 and MTL650 displays/terminals (as
an alternative to the MTL696 communications interface) see the appropriate product instruction
manual.
Table 10 .2
MTL5051
Terminals
MTL640
mode
MTL650
mode
Comms
mode
Other IS
devices
1
Common
Common
Common
Common
2
V signal
12V
-
5V/12V
3
I return
Rx
Rx
-
4
-
Tx
Tx
-
5
-
-
-
Tx
6
-
-
-
Rx
Switch
1a
On
Off
Off
Off
1b
On
On
On
Off/On
Table 10 .3
Terminals
RS232 mode
TTL mode
RS422 mode
7
-
-
Rx-
8
-
-
Rx+
9
-
Tx
Tx+
10
Tx
-
Tx-
11
Common
Common
Common
12
Rx
Rx
-
13
Supply -ve
Supply -ve
Supply -ve
14
ve
ve
ve
Switch
2a
Off
On
On
2b
On
Off
Off
Across hazardous areas: For communication across hazardous areas MTL5051 devices are used
in pairs to transfer bi-directional full duplex data across hazardous areas, as shown in figure
10.4. Current switching is used to minimise the bandwidth-limiting effects of long cables.
The maximum baud rate in this mode is the lesser of 19.2k baud or the cable-related rate
produced by the following formula.
Remote signalling baud rate formula, for back-to-back mode across a hazardous area:
max baud rate = K/(RxCxL2)
where
K = 0.25 (constant)
R = cable resistance (Ω/m)
C = cable capacitance (F/m)
L = length (m)
For example, with a 2km cable of 100pF/m capacitance and 40mΩ/m resistance, the maximum
baud rate = 0.25/(40mΩ x 100pFx 2km2) = 15k baud. This assumes that the cable is 2 cores
plus screen, with the screen used for the ‘common’ connection.
RS232-level devices:
Communication with RS232-level interfaces, such as a suitably certified
IS keyboard, mouse, etc, is achieved by using one or more MTL5051 units as required by the IS
device.(TTL level interfaces are accommodated by the TTL compatibility of RS232 receivers.)
The supply to IS equipment at terminal 2 can be set to either 5V or 12V, by a switch located on
top of the unit, as follows:
+12V mode
12.0V ±5% (load <23mA)
+12V mode
8.0V min (load >23mA to <50mA)
+5V mode5.6V ±5% (load >23mA to <50mA)
Note:
the normal RS232 limitations of bandwidth versus cable length are applicable. As a rule
of thumb, speed (baud) x length (metres) <150,000.
Figure 10 .5:
Top label
for MTL5051
