Protocol Configuration Values

	NOTE: Applicable only in Loop Mode; default value is 100 ms.

This parameter is used to configure an extra delay in the poll cycle when Loop Mode is in use. The parameter values currently supported are as shown below.

This parameter defines the address which is used to communicate with a secondary station. This parameter is required for both primary and secondary HDLC-NRM (SDLC) terminals. For a primary terminal, it is still referred to as the secondary station address, because the address belongs to the secondary station.

This secondary station address has a valid range of 0x00 to 0xFF. The broadcast terminal address (if defined) should be set to 0xFF. Each terminal defined on a line requires a different address.

In the case of a primary station this bit determines the resolution which applies to the T1 parameter as follows:

In the case of a secondary station this bit becomes the top bit of the “Poll Timeout” parameter.

This parameter is used to set the T1 response timer for a primary configuration or the Poll Timeout for a secondary configuration.

For a primary terminal, the T1 response timer value is set by a combination of the T1R timer resolution parameter and the T1 poll timeout value. The value of the timer should be chosen so that it is longer than the time taken to transmit the longest I-frame. The T1 timer is started after a frame with the poll is transmitted and is restarted after each I-frame that does not contain the final bit is received. A detailed explanation of the T1 timer processing is given in Chapter 3, “Response Timer”.

The user may choose a value for the T1 parameter between 2 and 31. A value of 1 is not recommended as it can lead to an arbitrarily low timeout value. The default value for the poll timeout value (if both T1 and T1R parameters are zero) is one second.

When choosing a value for the timer, care should be taken to ensure that the internal rounding down of the timer value, which occurs at run time, does not lead to a shorter than expected timer value. This can be achieved by either choosing a higher resolution (100ms), if possible, or by choosing a value of the T1 parameter which is one unit larger than the minimum desired timer value.

Table 4-1 “Timeout Values” shows the minimum and maximum actual timeout values which will be achieved with various combinations of T1R and T1 parameters.

For the secondary Poll Timeout, the configured units are 1 second, but this timer has a 10 second resolution. The default (if the parameter is set to zero) is 20 seconds. This parameter is used to detect a disconnected condition for the secondary station, where a poll is not received for the duration of this timer. For the secondary, the allowable values for this timer are 1 to 63 seconds.

This parameter is used to set a limit on the size of the received I-field of a frame that will be allowed. This parameter does not restrict the size of I-frames which may be transmitted.

This limit does not include the Address byte, Control byte or FCS characters. The parameters correspond to byte lengths as follows:

This is the maximum number of unacknowledged transmitted I-frames allowed. Allowable values are 1 to 7 inclusive. The default (if this parameter is set to zero) is one (1). This parameter is applicable to both the primary and secondary stations.

For the primary broadcast terminal, this parameter can be set to govern the number of broadcast messages that will be sent on a poll cycle.

Increasing this parameter will allow for higher throughput where there are multiple messages to send, because acknowledgments are not required for every message transmitted. This parameter is normally set in conjunction with the capabilities of the receiving station to receive multiple messages. On a line with many terminals defined, or with large messages sizes to transmit, the use of a large window size will require large numbers of transmit buffers to be used. When configuring a large window size, the terminal unacknowledged queue limit may need to be increased in order to take advantage of the window.

This is the retransmission retry limit. The default (if this parameter is set to zero) is seven (7).

This parameter governs the number of times the protocol will attempt to retry after an error condition, before resorting to a higher level recovery action.

Set to disable the normal slow poll facility for a primary station when the terminal is down. When set for a primary station, the station will be polled once during each normal poll cycle, even when down. For further discussion of the slow poll mechanism, refer to the “Response Timer” section of Chapter 3. This option is only applicable for a primary station.

Set to enable loop mode operation. Refer to Appendix B “SDLC Loop Mode Option” If this option bit is set (1), then the HDX (half duplex) bit should also be set (1), and all other option bits clear (0).

Set to make a terminal the broadcast terminal for the port. A single broadcast terminal may be configured on either a primary or secondary port. A broadcast terminal's address should be set to 0xFF.

The broadcast facility is an extension feature of the HDLC-NRM (SDLC) basic procedures as described in ISO 4335. The broadcast facility allows the primary control station to send messages without acknowledgment to all secondary addresses on a line (if these are capable of receiving such messages).

For a line configured for secondary operation, a secondary broadcast terminal may be configured to allow the reception of any broadcast messages sent by the primary control station. In this implementation, the secondary broadcast terminal only will receive broadcasts, and not any of the other secondary stations.

For operation with the broadcast facility, a terminal ZLU should be configured with the all stations address “11111111”, (0xFF) in addition to the real terminals. Messages to be broadcast should be sent to this virtual terminal ZLU by an application program. On the secondary line, all broadcast messages will be received by the secondary broadcast terminal.

The virtual broadcast terminal is distinguished by this BCT option setting which will change the way in which this terminal will be handled by the protocol as follows:

A broadcast message will be sent as an unnumbered information (UI) command with P bit set to zero (0). No response is expected to the broadcast message from any secondary station.

This option flag may be set to enable the control of the RTS signal as frames are transmitted. This option must be set the same for all terminals on the port.

If set to zero, the RTS control signal is left permanently ON after the port is enabled.

This option is only applicable for the primary stations. The setting of this option for a primary terminal will prevent the transmission of frames to ANY SECONDARY TERMINAL while the poll is outstanding to a terminal configured as half duplex.

If the half duplex option flag for a primary terminal is zero (0), then the terminal will operate in full duplex mode.

Set to invoke support for the remote power control feature for the station. Stations do not support remote power control by default. This procedure is outside of the standard procedure for HDLC-NRM (SDLC), as defined in ISO 4335.

This option is only applicable for primary stations. Secondary stations will respond as expected to the RPC sequence if received, without setting this configuration option.

The RPC command is defined as a UI command frame, with empty I-field and P bit equal to “1”. The RPC procedure operates outside of the normal mode setting procedure using the SNRM command.

When the RPC is enabled, the primary station will initiate the RPC command for a station when the terminal is enabled. The primary station will continue to send the RPC command until either a DM or FRMR response is received. The primary station will then proceed to set the mode using the SNRM command.

Set to enable two way simultaneous operation of a station. This option is only configurable for primary stations. Primary stations operate in two way alternate mode if this option is set to zero (0). Secondary stations will operate in TWS if permitted by the discipline enforced by the corresponding primary station.

The setting of the TWS option will allow frames to be sent to the secondary station while the poll is outstanding to that station.

	NOTE: Use of the TWS option is not recommended when there are multiple polled primary terminals configured on the line (i.e. multipoint rather than point-to-point). This is for the reason that, when a secondary terminal has high volumes of data to send, the primary station keeps acknowledging received frames, consequently allowing the secondary an arbitrarily long time to return the final bit to the poll. Since in the TWS mode the interval between polls is indeterminate, using the TWS option under these conditions is not recommended.

Set to make the terminal a secondary station. If this option flag is set to zero (0), then this terminal is configured as a primary station. Primary and Secondary terminals cannot be mixed on a single port.