Westermo MD-21 - Manual (Page 7)

Hints
The 20mA current loop interface, or TTY as it is sometimes known, is a
popular industrial communications standard. The system relies on a cur-
rent generator running on both the transmit and receive circuits. On each
circuit it is important to have only one current generator supplying cur-
rent into that circuit. For this reason the MD-21 can have its current gen-
erators set to be either active or passive. It is important to check the state
of all attached equipment to ensure correct setting on the MD-21. A maxi-
mum of 6 devices can be connected in a loop.
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 recomendation of cable configurations are
given below.
If any problems do occur on set up of the MD-21, the LED’s will be help-
ful.
- PWR: The unit has power.
- RD: Data received on line interface.
- TD: Data received on RS-232 interface.
A good way to check the MD-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 both passive. Connect T+ to R+ and T- to R-. Connect the
RS-232 interface 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.
13 6151-2002
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DTE MD-21 D-sub
DCE MD-21 D-sub
9-pol. PC MD-21 D-sub