Flow Control Settings for Serial Devices

1. Data Terminal Equipment (DTE) and Data Communication Equipment (DCE)

Systems COM ports, Digi C/X concentrator ports and terminal ports are considered Data Terminal Equipment devices (DTE) while a modem is considered a Data Communication Equipment device (DCE).

Assuming that the COM ports, C/X concentrators and terminals ports have DB-25 pin connectors:

• To connect to another DTE device a reversing cable is required.
• To connect to DCE devices a straight through cable is required.

For terminals, their Data Terminal Ready (DTR) outgoing signal is used for hardware flow control.
For serial printers the Printer Busy outgoing signal is used for hardware flow control.
For modems the RequestToSend/ClearToSend (RTS/CTS) signals are used for hardware flow control.

A Note on Terminology.

The terms input and in, and output and out are used in this document to indicate the direction of data flow with respect to the mvBase system. Data being displayed on terminals, or printed on printers, or sent over modems is considered output. Data being entered from a keyboard or being received from a modem is considered input.

Flow control can be specified for data flows in either, or both, directions. Thus In Flow refers to controlling the flow of data being input to the system, and Out Flow refers to control the flow of data being output from the system.

Hardware flow control signals are considered incoming or outgoing with respect to the individual devices. Thus the DTR signal line attached to the DTR pin on the connector on a terminal is an outgoing signal with respect to the terminal. It is normally connected to the CTS pin on the connector on the DTE device and thus becomes the incoming CTS signal with respect to the DTE device (Systems COM ports, Digi C/X concentrator ports and terminal ports).

2. Wiring

2.1 Wiring Terminals and Serial Printers for Hardware Flow Control

To wire a terminal or serial printer which supports DTR (or printer busy on serial printers) hardware flow control, the terminal's outgoing DTR signal (or serial printer's printer busy signal), usually pin 20, should be connected to the DTE device's (system COM ports, Digi C/X concentrators, etc.) incoming CTS signal pin.

For 9-pin system COM ports connector the incoming CTS signal pin is pin 8, for a DB-25 connector this is pin 5. For the Digi C/X concentrators RJ-45 the incoming CTS signal pin is pin 8.

The outgoing RTS signal pin of the DTE device (system COM ports, Digi C/X concentrators, etc) should be connected to the terminal's incoming CTS signal pin 5.

For system COM ports the outgoing RTS signal pin is normally pin 7 on a 9-pin connector, pin 4 on a 25-pin connector.
For the Digi C/X RJ-45 connector the outgoing RTS signal pin is pin 3.

Note: Port communication configuration changes does not require mvBase Workstation shutdown. However to avoid a license usage for a serial printer, you must check the serial printer box in the Port Info field, which does require a shutdown and restart of the workstation.

2.2 Wiring Modems for Hardware Flow Control (and connecting two system COM ports back-to-back).

Most modems by default use hardware flow control (RTS/CTS). For 9-pin COM ports, a DB-9 female to DB-25 male modem cable is required. For DB-25 pin COM ports, a straight through cable is required. System's COM ports and Digi C/X concentrators support RTS/CTS hardware.

When connecting between COM ports on two systems a 'full' reversing cable is required.

3 Configuring mvBase for Flow Control

3.1 Using the mvBase Administration Utility

Figure 1 shows a typical mvBase Port Configuration screen. This can be entered by launching or switching to mvBase Administration Utility. Select the Workstation tab. Select Configure. Select Port tab. By default, the XIN, XOUT, RTS and DTR state boxes are checked to support software flow control.

Figure 1

Flow Control Options

There are eight Flow Control checkboxes which can be set on a port-by-port basis. Although it would appear from the lack of visual grouping that there is no interdependence between them, not all of the possible 256 combinations are either legal or meaningful. Each check box is described below, followed by an explanation of the logical groupings.

There are two directions that flow control can be specified for - In Flow Control (or Input Flow Control) refers to controlling the flow of data in to the system from a terminal or serial printer. Out Flow Control (or Output Flow Control refers to controlling the flow of data from the system to a terminal or serial printer. Typically, one method of Input Flow Control and one method of Output Flow Control would be specified. The descriptions below are from the point of view of a system that is connected to a terminal, model or serial printer device.

Software (in band/XON-XOFF) Flow Control Options

• XIN Flow - when checked, the system is the initiator of sending Control-S (ASCII DC3) to halt input from the terminal or modem into the system, or Control-Q (ASCII DC1) to resume it. Thus XIN flow is describing controlling input to the system, which is coming from the terminal or modem.

• XOUT Flow - when checked, the terminal or serial printer is the initiator of sending Control-S to halt output to the terminal or serial printer from the system, or Control-Q to resume it. Thus XOUT flow is describing controlling output from the system, which is going to the terminal, modem or serial printer device.

Hardware (signal-based) Flow Control Options

• RTS In Flow - (similar to XIN flow but using the RTS signal pin instead of the XON/XOFF software flow control mechanism). When checked, the system's outgoing RTS signal controls the flow of data being input to the system from a terminal or modem. When the signal is high, it indicates that the system is ready to receive data, when the signal is low it indicates that the system is not ready to receive data.

• CTS Out Flow - (similar to XOUT flow but using the CTS signal pin instead of the XON/XOFF software flow control mechanism). When checked, the incoming CTS signal from the terminal, modem or serial printer device controls the flow of data being output by the system to the terminal, modem or serial printer device. When the signal is high, it indicates that the terminal, modem or serial printer is ready to receive data, when the signal is low it indicates that the terminal, modem or serial printer is not ready to receive data.

• DTR In Flow - (similar to RTS In Flow, but using the DTR signal pin instead of the RTS signal pin). When checked, the system's outgoing DTR signal controls the flow of data being input to the system from a terminal or modem. When the signal is high, it indicates that the system is ready to receive data, when the signal is low it indicates that the system is not ready to receive data.

• DSR Out Flow - (similar to CTS Out Flow, but using the DSR signal pin instead of the CTS signal pin). When checked, the incoming DSR signal from the terminal, modem or serial printer device controls the flow of data being output by the system to the terminal, modem or serial printer device. When the signal is high, it indicates that the terminal, modem or serial printer is ready to receive data, when the signal is low it indicates that the terminal, modem or serial printer is not ready to receive data.

• RTS (hi) State - when checked will keep a constant high signal on the outgoing RTS signal pin thus indicating to a terminal or modem which is observing that signal line that the system is always ready to receive data.

• DTR (hi) State - when checked will keep a constant high signal on the system's outgoing DTR signal pin thus indicating to a terminal or modem which is observing that signal line that the system is always ready to receive data.

Checking both the RTS State and DTR state checkboxes indicates that even if the terminal or modem recognizes these hardware flow control signals from the system, the system will always indicate that it is ready to receive incoming data. The use of XIN software flow control may then be combined with these RTS and DTR State options to allow the system to control the input of data to it by sending XON/XOFF characters to the terminal or modem.

RTS State and RTS In Flow are mutually exclusive, as are DTR State and DTR In Flow.

It is possible, but unlikely, for a single serial device (i.e. one device attached directly to one COM port) to require both CTS Out Flow and DSR Out Flow to be simultaneously specified. Likewise, for RTS In Flow and DTR In Flow.

3.2 Using the TCL PROTOCOL verb

The flow control settings in mvBase port configurations can also be changed using the PROTOCOL command. This is only effective when the line being configured is currently connected. Currently there is no support to set the DTR In Flow or DSR Out Flow boxes. p# is the process number.

4 Configuring the Terminals and Serial Printers in the mvBase Administration Utility

To configure a terminal for hardware flow control, clear XIN Flow, XOUT flow and RTS state boxes. Check the RTS Flow In and CTS Out Flow box. See Figure 2.

In the mvBase Workstation tab, Ports configuration, uncheck XIN Flow, XOUT flow and RTS state boxes. Check RTS In Flow and CTS Out Flow boxes. Click OK. A warning message will be displayed "Changing the protocol will change the actual hardware settings. Are you sure?". Click Yes to save the settings.

Changes can be also made in the mvBase port setting configuration using the above PROTOCOL command (described above).

Figure 2

To configure a serial printer for hardware flow control, clear XIN Flow and XOUT flow boxes. Check the CTS Out Flow box. See Figure 3.

Figure 3

5 DTR/DSR signals

The Data Terminal Ready (DTR) and Data Set Ready (DSR) check boxes in the mvBase Workstation configuration can be set providing the I/O controller installed supports these signals for hardware flow control.

From the point of view of the DTE equipment (system COM port, serial terminal adapter etc) the incoming DSR signal is treated like the incoming CTS signal and the outgoing DTR signal is treated like the outgoing RTS signal in the mvBase Workstation Port Configuration window screen.

5.1 Using DTR/DSR signals flow control with Terminals

To connect a terminal with hardware flow control, the outgoing DTR signal pin from the terminal would be connected to the incoming DSR signal pin of the system's I/O controller connector.

The I/O controller connector's outgoing DTR signal pin would be connected to the terminal's incoming CTS signal pin.

In the mvBase Workstation tab, Ports configuration, clear XIN Flow, XOUT flow and DTR state boxes. Check DTR In Flow and DSR Out Flow boxes. Click OK. A warning message will be displayed "Changing the protocol will change the actual hardware settings. Are you sure?" Click Yes to save. See Figure 4.

Figure 4

5.2 Using DTR/DSR signals flow control with Serial Printers

To connect a serial printer with hardware flow control, the outgoing printer busy signal pin would be connected to the incoming DSR signal pin of the I/O controller connector.

In the mvBase Workstation tab, Port Configuration, clear XIN Flow and XOUT flow boxes. Check the DSR Out Flow, DTR In flow and serial printer boxes. See Figure 5.

Figure 5