RS-232 Serial Interface

GENERAL DESCRIPTION OF RS-232

RS-232 serial communications uses fixed voltage values and generally allow you to connect to only one device (such as one temperature controller). These fixed voltage signals are more easily affected by noise and distance problems then are the differential signals of RS-485 serial communications, but with appropriate cabling you can run a cable 10 feet or more.

Our RS-232 interface also includes a protocol capability so that information can be sent to, and received from, the attached device. A protocol defines the way you encode commands and data for transmission and reception.

The protocol we use for our standard temperature controllers is unique to our controllers, and our custom built temperature controllers will sometimes be assigned new, custom protocols based on the requirements of the application. Our communications protocols apply to all our computer compatible temperature controllers regardless of the method of communications (RS-485, RS-232, GPIB, etc.)

For programming information for the protocol of our standard controllers see our Communications Protocol page.

PORT SETTINGS

Windows system device properties for serial ports...

For most of our standard Controllers the Port Properties' Port Settings are:

Bit per Second: 9600
Data Bits: 8
Parity: None
Stop Bits: 1
Flow Control: None

See your temperature controller's Operators Manual for different settings required by your controller.

CABLING

Normally, cable runs are not long and induced noise is not a problem. Telephone cable can be used for a cable.

Modular Connection: We can provide a 7 foot, 6 wire telephone cable with modular connectors on the ends. We can also provide a DB9 connector for this cable. To connect to the controller, you can either cut off a connector on one end of the cable and connect with bare wires, or you can use a Radio Shack® Snap-In Module Category 3 Jack, P/N 278-2022 which has a modular jack on one side and bare wire connectors on the other (bare wire connector #2 goes to the controller's JP3-2, #3 to JP3-1, and #4 to JP3-3).

MODULAR CONNECTION

Off-the-Shelf Cable: You can purchase a standard RS-232 cable and make up an appropriate mating connector with short connecting wires for the controller (connect Tx to Tx, Rx to Rx, and Shield to Common).

Custom Cable: Below are the connections you use to connect from a serial communications connector to a controller using the cable and cable length of your choice.

The DB9 and DB25 wiring information below is for our standard controllers, which at the time of this writing would be the 5C7-250, 5C7-251, 5C7-252, 5C7-362 and 5C7-365. For other controllers, check the instructions in your Operators Manual.

DB9: With a DB9 9 pin male connector for the computer:

The controller's JP3-1 goes to DB9 pin 3. (Rx to Tx)

The controller's JP3-2 goes to DB9 pin 2. (Tx to Rx)

The controller's JP3-3 goes to DB9 pin 5. (Shield to Common)

DB25: With a DB25 25 pin female connector for the computer:

The controller's JP3-1 goes to DB25 pin 2. (Rx to Tx)

The controller's JP3-2 goes to DB25 pin 3. (Tx to Rx)

The controller's JP3-3 goes to DB25 pin 7. (Shield to Common)

Cable Specifications: EIA RS-232 specifies:

Cable Configuration: Twisted Pair

Gauge: 24AWG (24 Gauge)

Nominal Shunt Capacitance per Foot Between Conductor Pairs: 15.5 pF (Picofarrads)

Nominal Shunt Capacitance per Foot Between a Conductor and the Shield: 15.5 pF (Picofarrads)

Nominal Conductor Impedance: 24 Ohms per 1000 feet

Shield type: Overall braided with drain wire (see paragraph below)

Maximum Cable Length: 10 feet when using 56000+ baud serial communications rate (see paragraph below)

Recommended: Approved by Underwriters Laboratories (UL)

Length: At the 9600 baud communications rate of our standard temperature controllers (such as the Models 5C7-250, 5C7-251, 5C7-252, 5C7-362 and 5C7-365) a properly configured cable can often be run up to 50 feet. A high quality cable may even allow lengths of over 100 feet.
Shielded Wire: Troubleshooting an operational problem that is due to cabling can be difficult. Since shielded wire is not much more expensive than unshielded wire, and since it offers improved noise immunity along the length of the cable, it is best to use shielded twisted pair (STP) wire for the cable. Connect the shield to the "Shield" connection of the controller, but if your controller does not have a shield connection then at least make sure the shield of the loop is connected from one section of cable to the next. Your computer's RS-232 serial port, or your converter, should also have a shield connection marked "Common Ground" (C or GND) or "Shield".
On the Bench: For testing/setup with very short runs (a couple of feet max) on a bench just about any wire can probably be used. However, with our standard temperature controllers, you will find it is easier to work with solid copper wire (similar to Radio Shack® 24 Gauge, 2-conductor Rainbow Wire, P/N 278-857) simply because it is easier to use with the euro style screw down connectors on the controllers. If you start testing for noise immunity, make sure the cable is up to spec.

TROUBLESHOOTING

If communications is not working properly check all the connections. Then check the following (note that a Comm port is a serial communications port on your computer):

For a Comm Port Timeout:

No power to controller, or converter (if any).

Device address is wrong. If you are using a device address in your communications with an RS-232 based controller (as you might from a custom program or when using a device driver) the address of the controller is one (1). The "universal address" of zero (0) can also be used.

Wrong Comm port is selected to access from your computer.

Comm port is not really connected to the device, or to the converter (if any), or the converter is not connected to the device.

Wiring to Comm port of computer, or converter (if any), or device, is incorrect.

For a Comm Port Open Error: No Comm port is actually available on your computer at the Comm port setting you selected.

Power Supply Voltage: Most controllers need 12 or more volts (check the specification for your controller). If the power supply is not "stiff" enough to support the controller(s) at 12 vdc communications will not function properly. A switching power supply rated at 12 vdc will compensate for the draw of the controller in order to maintain 12 vdc. A linear power supply rated at 12 vdc may dip under 12 vdc when powering the circuit board, or when the demand from any load (TEC or heater, etc) it is powering is to high. (Note that the common "battery eliminator" is generally a linear power supply.)