15
© Sealevel Systems, Inc. 3502 Manual | SL9116 9/2022
Appendix C – Electrical Interface
RS-232
Quite possibly the most widely used communication standard is RS-232. This implementation has been
defined and revised several times and is often referred to as RS-232-C/D/E or EIA/TIA-232-C/D/E. It is
defined as “
Interface between Data Terminal Equipment and Data Circuit- Terminating Equipment Employing
Serial Binary Data Interchange
” The mechanical implementation of RS-232 is on a 25 pin D sub connector.
The IBM PC computer defined the RS-232 port on a 9 pin D sub connector and subsequently the EIA/TIA
approved this implementation as the EIA/TIA-574 standard. This standard has defined as the “
9-Position
Non-Synchronous Interface between Data Terminal Equipment and Data Circuit-Terminating Equipment
Employing Serial Binary Data Interchange”
. Both implementations are in widespread use and will be referred
to as RS-232 in this document. RS-232 is capable of operating at data rates up to 20 Kbps / 50 ft. The
absolute maximum data rate may vary due to line conditions and cable lengths. RS-232 often operates at
38.4 Kbps over very short distances. The voltage levels defined by RS-232 range from -12 to +12 volts. RS-
232 is a single ended or unbalanced interface, meaning that a single electrical signal is compared to a
common signal (ground) to determine binary logic states. A voltage of +12 volts (u3 to +10 volts)
represents a binary 0 (space) and -12 volts (-3 to -10 volts) denotes a binary 1 (mark). The RS-232 and the
EIA/TIA-574 specification define two types of interface circuits
D
ata
T
erminal
E
quipment (
DTE
) and
D
ata
C
ircuit-Terminating
E
quipment (
DCE
).
The Sealevel Systems Adapter is a DTE interface.
RS-422
The RS-422 specification defines the electrical characteristics of balanced voltage digital interface circuits.
RS-422 is a differential interface that defines voltage levels and driver/receiver electrical specifications. On
a differential interface, logic levels are defined by the difference in voltage between a pair of outputs or
inputs. In contrast, a single ended interface, for example RS-232, defines the logic levels as the difference
in voltage between a single signal and a common ground connection. Differential interfaces are typically
more immune to noise or voltage spikes that may occur on the communication lines. Differential interfaces
also have greater drive capabilities that allow for longer cable lengths. RS-422 is rated up to 10 Megabits
per second and can have cabling 4000 feet long. RS-422 also defines driver and receiver electrical
characteristics that will allow 1 driver and up to 32 receivers on the line at once. RS-422 signal levels range
from 0 to +5 volts. RS-422 does not define a physical connector.
RS-485
RS-485 is backwardly compatible with RS-422; however, it is optimized for party line or multi-drop
applications. The output of the RS-422/485 driver is capable of being Active (enabled) or Tri-State
(disabled). This capability allows multiple ports to be connected in a multi-drop bus and selectively polled.
RS-485 allows cable lengths up to 4000 feet and data rates up to 10 Megabits per second. The signal levels
for RS-485 are the same as those defined by RS-422. RS-485 has electrical characteristics that allow for 32
drivers and 32 receivers to be connected to one line. This interface is ideal for multi-drop or network
environments. RS-485 tri-state driver (not dual-state) will allow the electrical presence of the driver to be
removed from the line. Only one driver may be active at a time and the other driver(s) must be tri-stated.
RS-485 can be cabled in two ways, two wire and four wire mode. Two-wire mode does not allow for full
duplex communication and requires that data be transferred in only one direction at a time. For half-duplex
operation, the two transmit pins should be connected to the two receive pins (Tx+ to Rx+ and Tx- to Rx-).
Four wire mode allows full duplex data transfers. RS-485 does not define a connector pin-out or a set of
modem control signals. RS-485 does not define a physical connector.
