| ??? 12/01/04 00:16 Read: times |
#82256 - Sounds like a loading problem Responding to: ???'s previous message |
Are there bus bias resistors in the circuit? What protocol are you running? Modbus RTU can exhibit bizarre symptoms if the bus is not biased correctly. It would help if you had an oscilloscope to 'see' what is actually happening. Another test is to put a continuous '1' on the bus and measure the voltages, then a '0' and measure the voltages. Add/remove devices and repeat the test. If it is a resistive loading problem, you will see the voltages decrease as you add devices. From memory, you need at least 200mV difference between the A & B (or + & - ) signals in order for the receivers to work. The documents Erik refers to will give a more concrete value. The PLC's I use have 4k7 bias resistors in each device - thus we can only have about 7 devices on a bus until the loading becomes too great. This maybe your problem also. |
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