RS-485 Serial Interface

General Description of RS-485
Cabling
Converters
Activating an RS-485 Loop
Troubleshooting
 
 

RS-232 serial communications uses fixed voltage values and generally allow you to connect to only one device (such as one temperature controller). The fixed voltage signals are subject to noise and distance problems. EIA adopted a new serial interface in 1978 called the RS-422. 

RS-422 uses two signal lines. These lines provide communications that is not based on fixed voltage but rather on a "differential" voltage. Using a differential voltage greatly improved the noise immunity of communications and increased the reliability of longer runs of cable. RS-485 is an enhanced version of RS-422. 

RS-485 specifications include the ability to connect to 32 devices. Modern RS-485 device drivers can often exceed this standard and connect to 60 or more devices. 

The RS-485 connection is a parallel connection: (+) to (+), and (-) to (-), all the way down the RS-485 chain of devices (temperature controllers). Although a third line called the "Common Ground" (C or GND) or "Shield" is sometimes used with RS-485, it is not always used with our temperature controllers. (The Model 5C7-461 does not have a connection for the shield. while the 5C7-36x series of controllers do have a shield connection. Refer to the customer drawing for your controller.)

RS-485 also includes a protocol capability so that information can be sent to, and received from, the attached devices. A protocol defines the way you encode commands and data for transmission and reception. 

At this time, there is no one protocol for RS-485. Some of the existing protocols have names such as MODBUS® ASCII and RTU.

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. 
 
 

Cable Specifications: Cables for RS-485 connections are based on EIA RS-422 specifications:

• Cable Configuration: Twisted Pair
• Gauge: 24AWG (24 Gauge)
• Shunt Capacitance per Foot: 16 pF (Picofarrads)
• Characteristic Impedance: 100 Ohm (see paragraph below)
• Last controller in loop should have termination resistor
• Maximum Cable Length: 4000 feet
• Recommended: Approved by Underwriters Laboratories (UL)

Characteristic Impedance: The Characteristic Impedance is the resistance of the wire at infinite length. The termination resistors allow the less than infinite length cables you use to look as though they are infinitely long, and this virtually eliminates reflected signal interference. Our controller documentation specifies 120 Ohm termination resistors. Use 120 Ohm characteristic impedance wire. (If you do use 100 Ohm charateristic impedance wire, use 100 Ohm resistors.) 

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-485 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 few feet) 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. You can ignore the shield connection unless you are using shielded wire in a noisy environment. As long as the run of cable on the bench is short, you can place the termination resistor at the head of the cable instead of at the end so that you can more easily switch controllers in the loop without having to deal with the resistor. If you start testing for noise immunity, make sure the cable is up to spec. 
 
 

RS232/RS485 CONVERTERS

RS232/RS485 converters allow you to connect an RS-485 loop to an RS-232 port, such as an RS-232 serial port on the back of most PC computers. Not all converters are equal, however. Extensive use of our converters by our customer base has shown our converters to be reliable even when other converters are not. 

Converters should meet or exceed the RS-422 signal specifications used with RS-485 communications: 

• Differential transmitted signals:
• +2 to +6 (for "Contorl A" or "+")
• -2 to -6 vdc (for "Control B" or "-").
• Receiver sensitivity: +/-0.2 vdc.
• Input impedence: 4 KOhm.
• Maximum input voltage: +/- 10 vdc.

We sell the IHV24AT-B9FSPS.

GPIB/RS485 CONVERTERS

We use the ICS Electronics converters along with a Kajei temperature controller device drivers. The Kajei device drivers uses the National Instruments VISA to communicate with the GPIB/RS485 converter. 

The following GPIB/RS485 converters have been used: 

4804A GPIB to Serial Interface Board. 

    4894A GPIB to Serial Interface

 
 

A "Loop" is a number of devices on the same RS-485 cable. 

When using the following instructions, if the "Software" (Setup Application) locks up for some reason (such as not having a controller powered) simply click the Software's "Closer" (the "X" button, upper right on title bar).

 

Begin with none of the controllers powered, but all controllers connected to the loop.

(1) Power up only one controller. 

 Leave the other controller(s) without power even though they are connected together via RS-485. 

 Leave the RS-485 termination resistor on the controller that is physically the last controller in the Loop. 

(2) Launch the Software if it is not already running. 

(3) In the PC COMMUNICATIONS group: 

 Select the COMM port. 

 Enter 0 (zero) as the ADDRESS OF CONTROLLER TO ACCESS. 

 Check "Enable New Controller Address" (So that a checkmark is displayed. I ALWAYS do this when entering a different value for the above, address of controller to access.) 

 Click the INITIALIZE button. 

You should now be connected to the one controller that is powered. 

(4) In the CONFIGURE group look at the value of CONTROLLER COMM ADDRESS. 
(5) Enter the CONTROLLER COMM ADDRESS value into the PC COMMUNICATIONS group's 
ADDRESS OF CONTROLLER TO ACCESS. 

 Check "Enable New Controller Address" (So that a checkmark is displayed. I ALWAYS do this when entering a different value for the above, address of controller to access.) 

 Click the INITIALIZE button. 

You should now be connected to the one controller powered up using it's own controller address. 

Follow the same steps above for a second controller and make sure the two have different addresses. 

If the two have separate addresses... 

(6) Power up only those two controllers.

 Leave any other controllers without power even though they are connected together via RS-485. 

 Leave the termination resistor where it is. 

(7) Launch the Software if it is not already running. 

(8) In the PC COMMUNICATIONS group: 

 Select the COMM port.

 Enter one of the two valid address numbers determined above as the ADDRESS OF CONTROLLER TO ACCESS. Do not enter 0 (zero) because 0 is the universal address and both controllers will respond simultaneously, talking on top of one another. 

 Check "Enable New Controller Address" (So that a checkmark is displayed. I 

ALWAYS do this when entering a different value for the above field, the address of controller to access.) 

 Click the INITIALIZE button. 

You should be in communication with one of the two controllers. 

Follow the above steps, 6 thru 8, and access the other controller. 

If the above works and you are able to access each of the two controllers... 

 Use steps 1 thru 5 to double-check that each of the remaining controllers has a unique address. 

 Use steps 5 thru 8 to access each of the 6 controllers. 

 
 

If communications is not working properly check all the connections. If you can limit the problem to one or two controllers it will help you limit the number of possible causes. Then check the following (note that a Comm port is a serial communications port on your computer): 

For a Comm Port Timeout:

1. No power to controller, or converter (if any).
2. Device address is wrong.
3. Wrong Comm port is selected to access from your computer.
4. Comm port is not really connected to the device, or to the converter (if any), or the converter is not connected to the device.
5. 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.)

Ground Loop: The existence  of a ground loop depends on how you have these controllers connected to power. It depends less on the communications cable if the run from one controller to another is not far. A ground loop occurs when the devices on an RS-485 loop do not share the same Ground (GND). This will not happen if one power supply powers all the devices. It can, however, occur if several power supplies are used for the devices on the loop. One way to compensate is to connect the device input power ground wires together to form a common input power ground.

If tracking down the problem is difficult...
Contact McShane