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RS-232 Troubleshooting Tips for TestEquity Chambers


We receive many calls each week regarding RS-232 communication issues. There are several issues which must be addressed in order to communicate successfully with the chamber. With thousands of chambers in the field, we have never had a verified case of a bad serial interface in the controller. Below are several of the most common communication issues.

Are you using the correct cable? 
The DB-9 connector is wired to accommodate a null-modem cable. A null-modem cable has pins 2 and 3 swapped on each end. You CANNOT use a straight through cable.

Is the baud rate set correctly? 
All TestEquity chambers shipped since mid-2003 have the baud rate set at 9600. Older chambers have the baud rate set at 19200. The baud rate of the controller MUST match the baud rate in your communications program. The baud rate is changed in the controller's Go to Setup\Communications menu. You may require a password to enter the Setup menu. If so, call TestEquity to get the password.

Does your software communicate using the Modbus RTU protocol? 
To communicate with the controller from a PC, you need to run software that uses the Modbus RTU protocol. You CANNOT use Hyperterminal. You CANNOT send simple ASCII or SCPI commands directly to the chamber's RS-232 interface.

Watlow Comm7 Utility Software
Comm7 (2.6M) is a Windows-based diagnostic utility created by Watlow. Comm7 can be used to upload/download controller configurations or log data to a text file via RS-232. Included in this download are files to restore the setup of F4 controllers back to the configuration as shipped from TestEquity.

Sample 16-bit Modbus Packet
Modbus_Packet.pdf (pdf, 209K) illustrates sample 16-bit Modbus packets to write a temperature set point and read the actual temperature.

Modbus Registers
Each controller function has a "register" number which can be read or written to (when applicable). These registers are listed Chapter Seven of the Series F4 User's Manual (pdf, 4M) .

Common Modbus Registers

  • The actual chamber temperature reading is Modbus register 100 (Input 1 Value).
  • The static temperature set point is Modbus register 300 (Set Point 1).
  • The temperature set point during a profile is Modbus register 4122 (Set Point 1, Current Profile Status).
  • The decimal points are implied. For example, 1005 is actually 100.5 and -230 is -23.0.
  • The Digital Output 1 (Event 1) function is Modbus register is 2000.