Meridian America Link/Customer Controlled Routing - Manual (Page 292)

274 Chapter 11: Peripheral device cabling interface
553-3202-210 Standard October 1998
¥ using a null-modem cable A null-modem cable (a special cable, or a
cable with an adapter) crosses certain RS-232 data signals so that a
DTE-to-DTE or DCE-to-DCE connection behaves like a standard
DCE-to-DTE connection. A null-modem cable is shown in Figure 80.
Note that null-modem cables and connectors are commonly used in
asynchronous connections (to terminals or modems, for example) but
cannot always be used in synchronous connections (to the Host or AML). In
particular, DTE-to-DTE synchronous connections cannot normally be used
because the DCE side supplies the clock signal, so that in a DTE-to-DTE
connection, no clock signal would be available. Likewise, DCE-to-DCE
connections may or may not work in synchronous mode because each side
thinks it controls the clock signal. Also, the port hardware implementation
may not allow a DTE-to-DCE crossover connection.
Figure 79
Straight RS-232 cable
Pin
1
2
3
4
5
6
7
8
20
15
17
Pin
1
2
3
4
5
6
7
8
20
15
17
DCE DTE
Note 1: Equipment at either end may require special configuration to
look like DTE or DCE so that a straight cable can be used.
Note 2: Pins 15 and 17 are used for some configurations.