Connect Tech Blue Heat/Net User’s Manual
Revision 0.13
107
Serial Line Interface Tutorial
RS-232 Line Interface:
RS-232 is the simplest, least expensive line interface standard. It is also referred to as EIA232
and TIA/EIA-232. The RS-232 specification signals levels of +3V to +15V for a logic 0 or
Space, and -3V to -15V for a logic 1 or Mark. The Blue Heat/Net has RS-232 signal levels with
a typical range of +/- 8 Volts. The maximum cable length you can use with RS-232 is dependant
on a number of factors including:
Baud rate
The faster the baud rate, the shorter the cable length must be.
Cable quality
Quality refers to the capacitance of the cable. A higher capacitance (usually specified as pF or
pico Farads per foot) dictates a lower baud rate, and a shorter maximum length. Low capacitance
computer cables for RS-232 applications are available from all wire and cable suppliers.
You can usually operate with cable lengths of up to 100 feet (30 m) at baud rates up to 115.2
Kbps using low capacitance cable. For higher baud rates such as 230.4 Kbps and up, we
recommend keeping the cable lengths to within 25 feet (7.6 m).
The TIA/EIA232 specification specifies two connectors: the DB-25 and the DB-9. Each of these
connectors has a standardized pinout. Many serial communication products offer RJ-45
connectors, including the Blue Heat/Net family. Note that there is no standard pinout for RS-232
on an RJ-45 connector. The following table specifies Connect Tech’s RJ-45 pinout.
RS-232 signals:
DB-9 Pin
(Standard)
RJ-45 Pin*
Signal Description
1 3
DCD
Data
Carrier
Detect
2 4
RX
Receive
Data
3 5
TX
Transmit
Data
4
7
DTR
Data Terminal Ready
5 6
GND
Signal
Ground
6
2
DSR
Data Set Ready
7
1
RTS
Request to Send
8
8
CTS
Clear to Send
9 N/A RI
Ring
Indicator
*Connect Tech Pinout.
Figure 55: RS-232 signals
RS-485 Line Interface:
RS-485, or TIA/EIA485, is a differential line interface standard capable of high baud rates over
long cables. RS-485 is fully compatible with RS-422; which is considered a subset of RS-485.
The use of differential transmitters and receivers ensures RS-485 communications are reliable
and robust. This means two wires are used to transmit or receive a signal. One wire carries the
true or non-inverted signal; the other wire carries the inverted signal. The non-inverted signal is
labelled with a (+) and the inverted is labelled with a (-). The RS-485 specification refers to the
(+) as “TXB” and the (-) as “TXA”.
