6
SYNCHRONOUS MODEM ELIMINATORS SME–422, –V.35, –530, –X.21
The pages that follow this one contain charts and diagrams that will help you
configure your SME:
• Tables 3-1 through 3-4 discuss the possible settings of the individual straps
(jumpers) inside each SME model. Use the straps to configure several
different options for your SME.
NOTE
Clock timing is supplied to each port (J1 and J2) in the form of a “send timing”
signal (derived from that port’s clock source) and a “receive timing” signal
(derived from the other port’s clock source). The straps that determine how the
SME gets its clock for each port are among those described in Tables 3-1 through
3-4. In the descriptions of these straps, the A position is called “internal,” the B
position is called “recovered,” and the C position is called “external.” When a
port’s timing is internal, the clock is the SME’s internal clock signal. When a port’s
timing is external, the SME gets the clock from that port’s “terminal timing” signal.
When a port’s timing is recovered, the SME gets the clock from the opposite port’s
“terminal timing” signal. For most applications both ports’ straps would be set to
A (internal clock). A small number of applications (those in which one device
supplies the timing across the line) would require one strap set to B and the other
set to C. Extremely few if any applications would require both straps set to C, or
one strap set to C and the other to A. The SME will not work at all if both straps are
set to B, or if one strap is set to B and the other to A. The CCITT X.21 standard
does not include a “terminal timing” signal, so SME–X.21 units won’t work if either
strap is set to B or C.
• Table 3-5 gives the possible settings of the data-rate switches S1 and S2
inside each standard SME model. Use these switches to set the SME’s data
rate to that used in your system. Tables 3-6 and 3-7 show the data-rate
settings for SMEs with special clock modifications (see the notes on pages
1, 14, and 15).
• Figures 3-1 through 3-3 show the printed-circuit-board layout for each
SME model.
Use the appropriate tables and figures to set your SME’s internal straps
and switches (substitute Table 3-6 or 3-7 for Table 3-5 if necessary):
• For the SME–422, use Tables 3-1 and 3-5 and Figure 3-1.
• For the SME–V.35, use Tables 3-2 and 3-5 and Figure 3-2.
• For the SME–530, use Tables 3-3 and 3-5 and Figure 3-3.
• For the SME–X.21, use Tables 3-4 and 3-5 and Figure 3-1.
3. Configuration
