Operating instructions

METTLER TOLEDO AT/MT/UMT Balances
75
The RS-232 Interface
This section describes the electrical features of the RS-232 interface and provides tips on
how to use it.
RS-232 is without a doubt one of the most widely used interfaces in the world of data
communication. Despite its great popularity, it is also the cause of much trouble and
frustration. The possible causes of most problems may be summarized in a surprisingly
short list:
• plugs don’t match
• handshake lines either don’t work or stop all data flow
• only garbled characters are received
• transmitted data does not appear at the other end of the cable
The ABC of RS-232
This section illustrates a few basic concepts of the RS-232 data transmission. If your prior
attempts to master RS-232 have been successful, you may safely skip the following section.
Many users are not aware that with RS-232 connections, as with the cables of the mains
power system, a distinction must be made between connectors and receptacles (somewhat
frivolously referred to as male and female plugs). If data leaves a device at pin 2 then it must
enter the partner device at the same pin. This way a simple wiring scheme is possible,
namely pin 1 to pin 1, pin 2 to pin 2, etc. In the RS-232 world, devices with male connectors
are called DTE (Data Terminal Equipment), whereas devices with female connectors are
known as DCE (Data Communication Equipment).
The cabling problem would not exist if all manufacturers would stick to these rules. As in
the well-behaved world of the 110 Volt cable, where safety against electrical shocks is
paramount, each cable would have a male connector at one end and a female connector
at the other. In reality, however, one might encounter data terminals with female and
computers with male plugs (sometimes even correctly so, as they behave like a terminal).
In such cases it is no longer possible to set up a working installation by just providing the
matching cables.