Westermo MA-21 - Manual (Page 6)

12 6021-2002
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The 20mA current loop interface, or TTY as it is sometimes known, is a popular
industrial communications standard. The system relies on a current generator running
on both the transmit and receive circuits. On each circuit it is important to have only
one current generator supplying current into that circuit. For this reason the MA-21
can have it’s current generators set to be either active or passive. It is important to
check the state of all attached equipment to ensure correct setting on the MA-21.
The MA-21 has the same line interface as MD-21, MD-29 and are hence compatible.
The RS-232 interface is configured as DCE (Data Communication Equipment). Most
printers, PC’s and terminals are set as DTE (Data Terminal Equipment). Some recomen-
dation of cable configurations are given below.
If any problems do occur on set up of theMA-21, the LED’s will be helpful.
• PWR: The unit has power.
• RD: Data received on line interface.
• TD: Data received on RS-232 interface.
A good way to check the MA-21 is to carry out a loop back test. Ensure that either the
transmitter or receiver are set to active, but not both active or passive. Connect T+ to
R+ and T- to R-. Connect the RS-232 port to a terminal. When keys are pressed on the
terminal you should receive the echo on screen. The TD & RD lights will both flicker
simultaneously as you press the keys.
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DTE MA-21 DTE MA-21 9-pos. PC MA-21