Epson 8900 Series - Manual (Page 195)

Communication Interfaces 10-4
The following figure shows simplified block diagrams of two
types of data-communication systems and their associated
communication interfaces.
The long-distance system typically uses telephone lines to transfer
data from the host to the printer. Because this system is limited to
bit-serial data-transfer operations, it is necessary that the host-to-modem
interface at one end and the modem-to-printer interface at the other
end be serial interfaces. The local system uses a parallel interface to
connect a local printer directly to the host.
Data-Transfer Operations
The data-transfer rate can vary from one system to another, depending
on such factors as the host data-transfer rate, the type of communication
interface used, and the distance over which the data is transferred. To
compensate for these transfer-rate variations, the printer provides a
receive buffer to temporarily store received data prior to printing it.
long-distance system
telephone
lines
RS-232-C
serial
interface
RS-232-C
serial
interface
modem host printer modem
local system
parallel
interface
host
local
printer
communication interfaces (simplified block diagrams)