Figure 6-38.--Rotary stepping switch selector.

busy mark and send a busy signal to the calling party

function of the allotter (fig. 6-37) is to assign

(station 78).

linefinders to any of the line circuits as required. When

any line requests service, the allotter assigns a

SELECTOR.-- The selector in the MDM

linefinder to the call, then preselects the next idle

200/700 system is connected back to back with a

linefinder to be assigned to the next call. As busy

finder (linefinder); that is, each finder has its own

linefinders become idle, they are made available for

associated selectors witch. The selector locates an idle

allotment to any subsequent calls. When one XY

connector in the group of lines (switchboard) to which

switch is connected to a given position in the wire

a call is being made.

bank, no other XY switch in the system may connect

The MDM 200/700 system uses a 20-point,

to that position. The S lead of the XY universal switch

8-level, rotary switch as a selector (fig. 6-38). This

and the S wire bank are used to indicate the busy

switch may be stepped by pulses from the dial or be

condition. For example, if station 55 calls station 21,

operated in an automatic hunt sequence by using

a busy indicator, called a mark, is immediately

interrupter spring contacts. The rotary switch wiper

extended to position 55 of the S wire bank from the

moves after it has been released electronically; that is,

station 55 line circuit. After the associated connector

when the step magnet is operated, the wipers do not

XY switch has been stepped to position 21 of the wire

go into the next position until the step magnet is

bank (under the control of dial pulses from station 55),

released. In a typical commercial XY installation, the

a busy mark is applied to that position in the S wire

rotary switch is used with the allotter circuits.

bank from the connector. If any other station, say

However, in the MDM system, the rotary switch is

station 78, now tries to call station 55 or 21, the

used in the selector circuits.

associated connector XY switch will encounter the

Figure 6-38.--Rotary stepping switch selector.

6-41