Universal Serial Port Cable Interfaces
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Universal Serial Port Cable Interfaces

The WSX automatically senses the cable type that you plug into one of its universal serial
ports. It can sense whether the cable type is DCE or DTE and whether it is one of the follow-
ing interfaces:
• RS-232
• RS-449
• RS-530
• V.35
• X.21 (European)
All cable types, except RS-232, are capable of access rates from 9.6 Kbps to 2 Mbps. The RS-
232 cable is not compatible with speeds greater than 64 Kbps. Each cable type is illustrated
and described in Appendix D, “Custom Cables.”
The WSX serial port is normally considered a physical DTE device. It is possible to turn it into
a physical DCE device simply by plugging in a DCE cable. The WSX board internally senses
whether a DCE or DTE cable is connected.
DTE/DCE Type and Transmit/Receive Pins
The RS-232 protocol, which is employed at the physical level for all cable types, always
defines Transmit and Receive pins in relation to the DTE. So, the type of cable you attach
(DCE or DTE) determines the direction of data flow on your connector’s Transmit and Receive
pins.
If the WSX serial port is a physical DTE, which is probably the most common configuration,
then data is received on Receive pins and transmitted on Transmit pins. If you are using a
WSX port as a physical DCE, then data is transmitted on the Receive pins and received on the
Transmit pins.

“Physical” and “Logical” Devices

This chapter refers to “physical” and “logical” DTE (Data Terminal Equipment) and DCE (Data
Communication Equipment) devices. A physical device operates on the network layer, and is
normally an actual piece of hardware, such as a WSX or CSU/DSU. Physical devices may
further be differentiated as DTE and DCE devices. A physical DTE device would be a piece of
hardware, such as a WSX, that does not control the access rate for virtual circuits. The physi-
cal DTE device is a conduit for data traffic but not a controller of data traffic. A physical DCE
device is hardware, such as a CSU/DSU, that does control access rates of Frame Relay traffic.
Normally physical DTE and DCE devices are directly connected to one another.
Logical devices operate on the Frame Relay protocol layer, and are sometimes referred to as
“Frame Relay logical” devices. Logical devices can also be broken down into DTE and DCE
devices. Logical DTE devices, again like the WSX, do not have direct control over the Frame
Relay network and the various congestion and control parameters that govern it. Logical DTE
devices do not control such actions as bringing up and tearing down virtual circuits; they act
upon updates and commands generated by the Frame Relay network. Logical DCE devices,
such as a Frame Relay switch, have a large span of control over Frame Relay network traffic.
They bring up and tear down virtual circuits, set congestion control bits in packets, and
communicate status to logical DTE devices.