The following guidelines apply when connecting an Operations Sentinel server through a terminal/communications server to managed systems or external alert devices:
The RS-232 standard defines certain aspects of making serial communication connections. The RS-232 standard is defined by the EIA. The title of the standard is “Interface Between Data Terminal Equipment and Data Communications Equipment Employing Serial Binary Interchange.”
The standard predates modern computers, which are sometimes used as termination equipment and sometimes as communications equipment, and often both at the same time. The standard defines the electrical signals at a particular port and not the actual role of the equipment.
The following electrical definitions are used:
To connect a serial device to an Operations Sentinel server through a terminal/communications server, you must know whether the device interfaces are DCE or DTE.
Cable Length Considerations
You are restricted by standard asynchronous cable (RS-232) to a distance of 50 feet. The 50-foot limit can often be exceeded if high-quality cable is used in a noise-free environment.
Note: These limits are the total cable length per port, including all cables used in a signal splitter connection.
DTE and DCE Pairing
Use the appropriate connections for pairing DTEs and DCEs as follows:
DTE connects to DTE through a null-modem cable or a 1-to-1 cable with a null-modem adapter.
DCE connects to DCE through a null-modem cable or a 1-to-1 cable with a null-modem adapter.
9-pin to 25-pin cables usually result in a DCE connection at the 25-pin end. This is used for connecting to a DTE device.
Use an RS-232 tester to ensure correct DCE and DTE pairings. It eliminates trial-and-error methods of using null modems in an undocumented connection environment.
Use of Null-Modem Adapter
Avoid using null-modem adapters by purchasing the correct connector types specified in the table in H.4.3. This reduces the number of electrical connections and increases reliability.
Test Tools
An RS-232 tester is useful in ensuring correct DCE and DTE pairing. Unisys recommends one of the following products from the Black Box Corporation:
Item QT-B, Quick Test Model B, Code TS001A-R2
Item QT-P, Quick Test B-Plus, Code TS009A
When pin 2 of the tester is green, the connection is DTE. When pin 3 is green, the connection is DCE. Using a tester can help eliminate trial-and-error methods of using null modems in an undocumented connection environment.