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Chapter 5 / The Electronics
ly. When the primary device has something to do, it initiates
Discovery. Discovery has two parts. They are:
• Link initialization
• Resource determination
The first step is for the primary and secondary devices to determine,
and then adjust to, each other’s hardware capabilities. These capabili-
ties are parameters like:
• Data rate

• Turnaround time
• Number of packets without a response

• How long to wait before disconnecting
Both the primary and secondary device begin communications at 9600
baud, which is the default baud rate. The primary device sends its
parameters, then the secondary device responds with its parameters.
For example, if the primary supports all data rates up to 115.2 kbaud
and the secondary device only supports 19.2 kbaud, the link will be
established at 19.2 kbaud.

Once the hardware parameters are established, the primary device
must determine if the secondary device has the resources it requires.
If the primary device has a job to print, then it must know if it is talk-
ing to a printer, not a modem or other device. This determination is
made using the IAS. The job of the secondary device is to respond to
IAS queries made by the primary device. The primary device must ask
a series of questions like:
• What is the name of your service?

• What is the address of this service?
• What are the capabilities of this device?
When all the primary device’s questions are answered, the primary
device can access the service provided by the secondary device.
During Discovery mode, the MCP2150 handles all responses to the pri-
mary device (see Figure 5.28) without any communication with the
host controller. The host controller is inhibited by the CTS signal of
the MCP2150 from sending data to the MCP2150.

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