Name: modem
Reference ID: NIST | USNO | PTB | GPS
Serial Port: /dev/modemu; 9600 8N1
Features: tty_clk
Requires: a dial-out device.


The ACTS mode of this driver reports only two-digit years, and is thus reliant on the system clock to be near correct before samples will be processed properly. You will not be able to use it to run autonomously, nor will it reliably recover from a trashed or zeroed system clock.


This driver supports the US (NIST and USNO) and European (PTB (Germany), NPL (UK), etc.) modem time services, as well as Spectracom GPS receivers connected via a modem. The driver periodically dials a number from a telephone list, receives the timecode data and calculates the local clock correction. It is designed primarily for backup when neither a radio clock nor connectivity to Internet time servers are available. It can also be configured to operate full period.

For best results the indicated time must be corrected for the modem and telephone circuit propagation delays, which can reach 200 ms or more. For the NIST service, corrections are determined automatically by measuring the roundtrip delay of echoed characters. With this service the absolute accuracy is typically a millisecond or two. Corrections for the other services must be determined by other means. With these services variations from call to call and between messages during a call are typically a few milliseconds, occasionally higher.

This driver requires a 9600-bps modem with a Hayes-compatible command set and control over the modem data terminal ready (DTR) control line. The actual line speed ranges from 1200 bps with USNO to 14,400 bps with NIST. The modem setup string is hard-coded in the driver and may require changes for nonstandard modems or special circumstances. It can be overridden by setting the extended system variable "modem_setup" via ntpq.

There are three modes of operation selected by the mode keyword in the server configuration command. In manual mode (2) the calling program is initiated by setting the flag1 option. This can be done manually using ntpq, or by a cron job. In auto mode (0) flag1 is set at each poll event. In backup mode (1) flag1 is set at each poll event, but only if no other synchronization sources are available.

When flag1 is set, the calling program dials the first number in the list specified by the phone command. If the call fails for any reason, the program dials the second number and so on. The phone number is specified by the Hayes ATDT prefix followed by the number itself, including the prefix and long-distance digits and delay code, if necessary. The flag1 is reset and the calling program terminated if (a) valid clock update has been determined, (b) no more numbers remain in the list, (c) a device fault or timeout occurs or (d) the flag1 option is reset manually using ntpq.

The driver automatically recognizes the message format of each modem time service. It selects the parsing algorithm depending on the message length. There is some hazard should the message be corrupted. However, the data format is checked carefully and only if all checks succeed is the message accepted. Corrupted lines are discarded without complaint. Once the service is known, the reference identifier for the driver is set to NIST, USNO, PTB or GPS as appropriate.

The Spectracom receiver can be connected via a modem if the radio is configured to send time codes continuously at 1-s intervals. In principle, the flag2 option enables port locking, allowing the modem to be shared when not in use by this driver. At least on Solaris with the current NTP I/O routines, this results in lots of ugly error messages.

The minpoll and maxpoll keywords of the server configuration command can be used to limit the intervals between calls. The recommended settings are 12 (1.1 hours) for minpoll and 17 (36 hours) for maxpoll. Ordinarily, the poll interval will start at minpoll and ramp up to maxpoll in a day or two.

US Phone Numbers and Formats

Note: Phone numbers include the entire Hayes modem command, including the ATDT and other control codes as may be necessary. For most cases only the ATDT may be necessary.

Phone: (303) 494-4774 (Boulder, CO); (808) 335-4721 (Hawaii)

National Institute of Standards and Technology
Telephone Time Service, Generator 3B
Enter question mark "?" for HELP
47999 90-04-18 21:39:15 50 0 +.1 045.0 UTC(NIST) *
47999 90-04-18 21:39:16 50 0 +.1 045.0 UTC(NIST) #

MJD, YR, ST, UT1 and UTC(NIST) are not used by this driver. The <OTM> on-time character "*" changes to "#" when the delay correction is valid.

Phone: (202) 762-1594 (Washington, DC); (719) 567-6743 (Colorado Springs, CO)

Data Format (two lines, repeating at one-second intervals)

jjjjj nnn hhmmss UTC

* on-time character for previous timecode message
jjjjj modified Julian day number (not used)
nnn day of year
hhmmss second of day

If a modem is connected to a Spectracom receiver, this driver will call it and retrieve the time in one of two formats, 0 and 2. Ordinarily, the receiver requires a T in order to return the timecode. As this driver does not send data via the modem, it must either be configured in continuous mode or be polled by another local driver.

Monitor Data

The received timecode is written as-is to the clockstats file along with the Hayes connection and hang-up commands and result codes.

Driver Options

unit number

The driver unit number, defaulting to 0. Used as a distinguishing suffix in the driver device name.

time1 time

Specifies the time offset calibration factor, in seconds and fraction, with default 0.0.

time2 time

Not used by this driver.

stratum number

Specifies the driver stratum, in decimal from 0 to 15, with default 0.

refid string

Set by the driver to (one of) NIST, USNO, PTB or GPS.

flag1 {0 | 1}

Initiate a call if 1. Automatically reset by program.

flag2 {0 | 1}

Enables port locking if 1, disables if 0 (default).

flag3 {0 | 1}

Not used by this driver.

flag4 {0 | 1}

Not used by this driver.


Not used by this driver.


Not used by this driver.

path filename

Overrides the default device path.

ppspath filename

Not used by this driver.

baud number

Overrides the default baud rate.

Configuration Example

refclock modem


David L. Mills <>

Additional Information