RS-232 Protocol
RS-232 is a serial communications interface standard. RS-232 was first  defined by the Electronic Industries Association (EIA) in 1962 as a  recommended standard (RS) for modem interfacing. The most current  standard, RS-232D (January 1987), was amended to be compatible with  international standards CCITT V.24, V.28, and IS2110.

RS-232 Pinouts
Standard RS-232 pin-outs for IBM compatible computers are shown below.  There are two configurations that are typically used: one for a 9-pin  connector and the other for a 25-pin connector.

The 25 pin connector has some pins that are not used for data transmission. These pins are mainly used for loop-back testing of the  port.
Note that the designations for pins 2 and 3 on the 9-pin connector are  the exact opposite of what they are for pins 2 and 3 on the 25-pin  connector.
Typically, A/V equipment will also utilize a 9-pin connector for  RS-232, though it is important to remember that the pin-outs will likely  vary from those on the control system/computer. For example, pins 2 and 3  may be reversed, allowing the interconnection to be made by a 9-pin to  9-pin cable with pin-to-pin wiring. More and more, A/V devices utilize a  "terminal block" type connector for RS-232. This type of  connector does not require soldering, allowing for a quick and easy  installation.

Voltage Ranges
The standard voltage range on RS-232 pins is _15V to +15V. This voltage  range applies to all RS-232 signal pins. The total voltage swing during  signal transmission can be as large as 30V. In many cases, RS-232 ports  will operate with voltages as low as _5V to +5V. This wide range of  voltages allows for better compatibility between different types of  equipment and allows greater noise margin to avoid interference.
Because the voltage swing on RS-232 lines is so large, the RS-232  signal lines generate a significant amount of electrical noise. It is  important that this signal does not run close to high impedance microphone  lines or audio lines in a system. In cases where you must run these types  of signals nearby one another, it is important to make sure that all audio  wires are properly shielded.

Terminal Block Connector

Baud Rate
In addition to voltage ranges, RS-232 Protocol identifies how fast data  is transmitted between two ports. This transmission speed is defined as  Baud Rate—roughly equivalent to the number of bits transmitted per  second. Typically, the baud rate will vary between 1200 to 19200. Common  baud rates are as follows: 1200, 2400, 4800, 9600, and 19200. Notice that  each baud rate indicated sequentially here is twice that of the previous  baud rate.

Cable Length Limitations
The length of the interconnecting cable between a control system and  controlled equipment must decrease as the baud rate increases. Typically,  at 1200 to 2400 baud, the cable length should be no more than  approximately 100 feet. At 9600 baud, the distance should be no more than  50 feet and, at 19200 baud, the cable length should be no more than 20  feet.

Troubleshooting
Problems with RS-232 connections can typically be separated into two  groups: those that are caused by hardware or physical hook-up conflicts  and those that are caused by software conflicts.

Hardware
Ironically, the most common problem that installers run into when  connecting a control system to a device results from simply not wiring it  properly.
The majority of control systems require the connection of only two  wires to the controlled device. The Transmit (XMT) and Ground (GND) pins  on the control system are connected to the Receive (RCV) and Ground (GND)  pins on the controlled device respectively, as shown below (Fig. 1).

In situations in which the control system needs to receive some type of  response from the controlled device, a third wire will also be connected  (Fig. 2). When using a computer to control a device through its COM port,  for instance, this is the recommended wiring.

Confirming a Proper Connection
Unless all of the pins are labeled, how can you be sure that you have  the made the proper wiring between the control system port and the  controlled device port?
If the controlled device uses a terminal block type connector, it is  quite easy to test the voltage using a voltmeter to ensure that the  connection has been made correctly. With the interconnecting cable in  place and with the voltmeter set to "DC", test the voltage  between the RCV pin and the GND pin on the terminal block connector.
The reading should be between _12V and _6V. The XMT line should also  have the same reading (Fig. 3)

If, after connecting the lines between the control system and the  controlled device, the voltage on the receive line stays at 0 volts, the  Receive and Transmit lines are probably reversed (Fig. 4).

Software
If you are still encountering problems after confirming a proper  hardware connection, you should then confirm the communications software  properties settings for both the control system and the controlled device.
Proper communication between the devices requires that the software  settings be set identically. For example, if the Baud Rate for the  controlled device is set to 2400, the control system must also be set to  2400.
Other parameters that must be set the same include Data Bits, Parity,  and Stop Bits. Data Bits define how many bits are in a single character  transmitted. This number can vary between 7 and 8. The Parity Bit defines  if the number of 1s during one byte transmission is odd or even. This bit  is set or reset to make sure the number of 1s in a single transmission is  always either odd or even. If the receiver does not care about Parity, it  can be set to None. The final Stop Bit defines the end of the transmission  of the data. This number can be 0, 1 or 2. Standard settings for these  parameters for most A/V devices are: 8, None, and 1 respectively (also  referred to as "8 n 1").

Using HyperTerminal
One way to troubleshoot problems with a third-party control system is  to first use a computer to ensure that the software settings have been  made correctly.
PC computers running the Microsoft Windows 95® or Windows 98®  operating systems offer a built-in application called HyperTerminal. This  application is typically accessible by selecting:

START>PROGRAMS>ACCESSORIES>HYPERTERMINAL
Selecting this opens a HyperTerminal Folder with several application  icons inside it. Select the icon named HYPERTRM.

When you launch HYPERTERMINAL, the first window that will pop up is  called Connection Description. This allows you to NAME the  connection and assign an ICON to it. Here we have named it "RS-232  Troubleshooting" and selected one of the icons available from the  window.

Select OK. Once you select OK, you may then receive a pop-up window named  Phone Number. Since you are not using a modem connection, you should  select "DIRECT TO COM 2" in the Connect Using field (assuming  that you are using COM Port 2 on your computer). Once you select this, the  Country code, Area Code, and Phone Number fields will become inactive.  Select OK.

You should then receive a pop-up window called COM2 Properties. This  window allows you to set the communications parameters as discussed  earlier.
Under normal circumstances, the baud rate should be set to the same  rate as the device that is being controlled and the other settings should  be as shown: Data bits: 8, Parity: None, Stop bits: 1, Flow control: None.  Select OK.

The final step is to configure the ASCII protocol. From the main  HyperTerminal window, Select FILE>PROPERTIES>
This will pull up the Properties window for your connection (in this  case, the RS-232 Troubleshooting Properties window). Select the TAB  labeled SETTINGS. On the SETTINGS window, you will see a button labeled  ASCII SETUP. Select this button. The following window will pop up.

In the ASCII Setup window, the "Echo typed characters  locally" box should be selected. This will allow you to see responses  in the main HyperTerminal window if they are provided. The "Wrap  lines that exceed terminal width" box should already be selected and  should remain selected. Select OK.
You should now have only the main HyperTerminal window open and all  settings should be complete. The device you intend to control should now  respond to ASCII commands as typed into the main HyperTerminal window.
Depending on the equipment being controlled, you may have to press the  ENTER key to execute a command. If the controlled device provides feedback  in ASCII text, and if you are using a three wire interconnect  (Transmit/Receive in both directions), you may also see responses from the  device you are controlling in the main HyperText window.

Main HyperTerminal Window