HOW THE RTS LINE IS USED WITH THE 5002

Only one device can talk on an RS485 network at a time. The RTS (request to send) line is used to put the 5002 into talk mode.

Fig 1: Computer talking to one remote device on RS485

Fig 2: Correct transmission and reception by PC

Correct operation of the bus

Look at the accompanying timing diagram (Fig 2) to see how a complete communication is carried out with a remote device.

The 4th line of the diagram is numbered. Each number represents a state of the RS485 line:

1. The RS485 line is at rest and is available
2. The PC raises the RTS line and puts the 5002 into transmit. No other device can use the bus now
3. The PC sends its data
4. Wasted time before the PC puts the RTS line low
5. The RS485 line is at rest and is available
6. The remote instrument responds. The data is transferred via the RS485 bus and is sent to the PC on its RXD line.
7. The remote instrument is finished with the bus and lets it go

Fig 3: Lost data due to RTS being held high too long

Incorrect operation caused by RTS being held high too long

If the PC is running a program such as Windows the RTS line may stay high too long. Please see the diagram above.

The 4TH line of the diagram is numbered. Each number represents a state of the RS485 line:

1. The RS485 line is at rest and is available
2. The PC raises the RTS line and puts the 5002 into transmit. No other device can use the bus now
3. The PC sends its data
4. The PC does NOT put the RTS line low immediately, and hence the RS485 line is not available.
5. The remote instrument responds and a collision occurs on the RS485 bus. The data is lost.
6. The PC finally puts the RTS line low, but it is too late.

This problem will not occur under DOS unless there are some long interrupts running in the system. Good windows drivers may also avoid the problem.

The 5001 or 5007 converters completely avoid this problem because they internally control when the units talk onto the RS485 line.