Serial Still Part of a Balanced Connectivity Diet

Most of the time today technical personnel are dealing with the new and ever-expanding areas of Ethernet and IP connectivity. The progress of PCs and network technology is moving so fast that younger technicians who are often IP literate find themselves sometimes getting caught off-guard when faced with some existing older connectivity technology that is still hanging around. PC technology that interfaces with various components still deals with serial and, yes, even in some cases, parallel communications.

In the past we have talked about this communication technology and I am expecting this may be the last time we report on it. However, it is still out there; it is a tried and proven technology. Besides, the manufacturers are still building serial com ports into the communication and control ports of devices such as access and CCTV controllers, and alarm signal receiver automation outputs. We still reference communication input/output (I/O) pinouts and standards such as RS-232 and RS-485. 

Serial Configurations and Functions

Serial port connections, depending on the number of control parameters, typically come in the 25-pin (DB25), 15-pin (DB15) or 9-pin (DB9) configuration. (“Tech Talk” trivia: The “D” subminiature connector indictor for “DB” is for 25-pin connector, or DB25. The class for 9-pin is actually a “DE,” or DE9, but it is often erroneously referred to as a DB9.)

The pins in serial communications connectors that always get the most attention are pins #2 and #3 for signal transmit and receive, and pin #7 (DB25) or pin #5 (DB9) for signal ground (not to be confused with shield ground). Also remember that RS-232 data is bipolar. A voltage level of +3V to +12V is an ON or 0-state (SPACE). A level of -3V to -12V is an OFF or 1-state (MARK).

While these are reference points for the serial signal, the other connections are important for some of the following reasons: 

Flow control — Pins #7 and #8, the process where one device asks another for permission to transmit data; also indicated as “clear to send” (CTS). 

Data terminal ready (DTR)/Data set ready (DSR) — Pins #4 and #6 indicate to start communications when the presence of active devices exists on both ends. 

Other — There are some other control levels and conditions, but I think you get the picture. For proper communications to take place, everything needs to be configured on both ends of the serial communication cable or channel.  When connecting devices for serial communications it should be identified if the device is data terminal equipment (DTE) such as a PC or data communication equipment (DCE) such as a modem. A pin #2 on a DTE device is transmit-out, while on a DCE it would be a receive-in. Normally this is not a big deal since one device transmits on one end and the   same conductor receives directly on the other end. But what if you want to have a DTE device communicate directly with another DTE device?

To do this the technician will need a specially configured “null modem” cable. In such a cable the transmit data (TD) and receive data (RD) connections are reversed. Additional handshaking connections may be required as well.  I recall my early security experiences using a null modem cable to connect a test PC directly to a remote access controls controller that normally would have been connected in the field via modems. Ironically, the null modem term is still used today with network DTE to DTE connectivity. Old terminology just keeps on ticking. 

Basic Serial Troubleshooting Tools

What happens if all hardware and connectivity efforts are made with a serial cable connection and communications are not happening? Some basic troubleshooting techniques can be applied.

First, double check all wiring connections with the manufacturers’ specifications. Next, you can use a   DVM to check if the device you are testing is DTE or DCE. Measure the DC voltage on pin #2 or #3 with reference to signal ground. If the voltage on pin #2 is more negative than -3V then it is a DTE. If the voltage on pin #3 is more negative than -3V then it is a DCE.

There are some key settings that need to be checked on both ends in order to make sure both devices are on an even playing field. The standard items to check are communication (COMx) port assignments, the speed or baud rate, parity bits, word size, and stop bits. These settings may be made either through software on a PC or printed circuit board jumper settings on field equipment.  Another way to double check if a serial port is functioning is the use of a loop-back plug. These devices can be homemade or store bought. The plug is wired in such a way as to basically loop signals out and then back into the same serial port. A very simple RS-232 loop-back plug configuration would have the transmit pin #2 and receiver pin #3 shorted.

Another troubleshooting tool is the use of a breakout box in which LED indicators indicate the communication status of pins on the serial connectors. There are also software programs that simulate breakout box indications.

The use of communications testing software can help not only to confirm communications but to check that the date being transferred is correct. Hyperterminal is a commonly available program that comes with all Microsoft™ operating systems. 

Taking Communications Beyond

So how do we take the existing and proven reliability of RS-232 serial communications and interface it with today’s demands of putting everything on a network?

Let’s say I   have a CCTV pan/tilt/zoom (p/t/z) controller that works using serial communications and I want to relay that RS-232 signal over an existing Ethernet in a facility. To do this I would want to look at the use of a serial-to-Ethernet server device, also sometimes called a PortServer. This can even be extended to the use of a wireless Wi-Fi interface.

A good example of this would be the Lantronix SCS400 server device (see diagram) that has a “modem emulation mode” allowing the capability to manage legacy equipment in an IP environment. Devices like these allow for the reliable elimination of plain old telephone system (POTS) connectivity with modems to Internet TCP/IP connectivity.

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