Flow Control

There are number of types of flow control

  1. None
  2. Xon/Xoff
  3. CTS/RTS Handshaking
  4. CTS/RTS Flow Control

 

Null Modems

    One thing to note about RTS and CTS is that, like send and transmit, what pin is what depends on if you are the sender (DTE) or the modem (DCE). Most devices that are designed to be connected to a computer are wired to be a modem. So if you are really talking to a modem you need toconvert the wiring, this is what a null modem does. The 37779 (Rev B) is an example of a null modem cable.

    All Trimble Survey receivers and the TSC-1 are configured as DCE's. The TSCe is configured as a DTE, which is why it needs a null modem to connect a PC.

     

None:

    No flow control is when the device and the modem assume that they can send or receive data at least as fast as it is being generated.

    No flow control is normally suitable for RTK rover applications. The receiver can nearly always keep up with incoming data at that rate.

    For most modems no flow control will work at the base station.

    Not all modems will allow you to run without the flow control, it is normally a setting in the modem to ignore flow control.

    Not all versions of survey controller support configuring the receiver for no flow control

     

Xon/Xoff:

    Xon/Xoff is a classical system that has been used with ASCII devices. When a device is about to have to much data it sends a Xoff character back to the device that is sending the data. The device will stop sending data until it gets a Xon character.

    While in theory you can send binary data using Xon/Xoff flow control it is not done. Most Trimble receivers do not support Xon/Xoff for this reason.

    Xoff is Ctrl-S, Xon is Ctrl Q, so if you have ever wondered why Ctrl-S would stop some outputs you now know.

 

CTS/RTS Handshaking:

    With CTS/RTS handshaking the receiver indicates that it would like to send data by asserting CTS (Clear to Send), the modem indicates that it is ready to send data by asserting RTS (Ready to Send). This is called handshaking it is a two stage process.

    Note that this is in terms of the recievers view of the world, the more normal way of saying it would be assert RTS (Ready to Send), wait for the Clear signal (CTS), which makes alot more sense to me,

    CTS/RTS handshaking has historically been used with dumb radios.With these radios the keying up of the radio to send data is based on the CTS line. The CTS delay was to give the radio time to key up.

    Modems do not use this form of flow control, they use the CTS/RTS Flow Control method

    The 4000/4400/4700/4800 do use this form of flow control.

    Since the older receivers and the modems do not use the same form of flow control the best thing to do is to not use flow control.

    Some versions of Survey Controller require flow control to be set on a port. In these case the only thing to do is create a cable that loops the CTS line from the receiver into the RTS line of the recevier. This means that when the receiver asks to send data it will always be allowed to.

    Note if you have a CTS/RTS loop cable connected to a recevier it will NOT BOOT, The looping of CTS/RTS is used to indicate that the receiver should go into monitor mode and await new firmware to be sent to it.

Name

Pin

Receiver Side

Phone Side

Notes

CD

1

Not Connected

Not Connected

 

Rx

2

Straight though

Straight though

If using a normal download cable cross over with 3

Tx

3

Straight though

Straight though

If using a normal download cable cross over with 2

DTR

4

Not Connected

Not Connected

Not Needed

GND

5

5

5

 

DSR

6

Not Connected

Not Connected

Not Needed

RTS

7

8 on receiver side

8 on phone side

Modem side optional if modem can be configured to work without flow control. Recommend to have no flow control if possible

CTS

8

7 on receiver side

7 on phone side

Modem side optional if modem can be configured to work without flow control. Recommend to have no flow control if possible

RI

9

Not Connected

Not Connected

Not needed, not recommended to be connected

 

    CTS/RTS Flow Control:

    With CTS/RTS Flowcontrol the terminal equipment asserts CTS when the modem is clear to send it data. The modem asserts RTS when it is ready to send data. This gives two way control, so the modem can indicate that it can't send data by not asserting RTS, the modem can send data to the terminal equipment at any time the CTS line is asserted

    You may have noticed that the above statement used the more formal term of termial equipment, instead of receiver and modem to be a bit clearer, since when used with a modem a null modem cable will be used

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