CHAPTER FOUR
I/O PORT DATA INPUT/OUTPUT
4.0 INTRODUCTION
The GPS timing unit has been equipped with an RS-232 interface using a nine pin connector
designated as RS-232 I/O J12. It can be used to communicate to and from the GPS ExacTime
unit. All communication is in the ASCII format. The standard character protocol is one start bit,
eight data bits, an odd parity bit, and one stop bit. However, the baud rate, number of data bits,
parity, and number of stop bits are selectable using the front panel keyboard switches. See
Chapter Three, “Additional Menu Screens.”
The interface cable pin assignments and designations are shown in Chapter Two.
All communication to the GPS ExacTime Unit consists of two or three categories of characters.
The first category is a single character, which is always an ASCII $ (Hex 24). This is the
attention/log-on character. The second category is an ID ASCII character, which is a command
to the GPS ExacTime Unit. The third category (which may or may not be applicable) is a series
of ASCII data bits to input data into the GPS ExacTime Unit. Leading zeros must be used where
necessary. For example, if the number is fifty-two, and if the data to be entered is a four digit
number, then it must be entered as 0052.
Note: If a mistake is made while inputting new characters (prior to the last character), issuing
the “$” character, this will cause a reset, and the new (correct) characters can be input.
When entering data via the RS-232 I/O port, if there is a pause longer than three seconds
between input values, communication with the GPS timing unit will terminate.
Table 4-1 shows the command options available. The ASCII character is shown following its
HEX equivalent. Following the table, each command is described with the necessary steps for
execution.
Table 4-1 is located on the following page.
Symmetricom Inc ET6xxx ExacTime GPS TC & FG (Rev C) 4-1
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