Data Transmission - IBM 1 Series Manual

Data Transmission
Data is transmitted and received by the teletypewriter adapterserially by
bit. Data is transferred one 8-bit character at a time. Each data character
is preceded by one start bit and is followed by one or two stop bits.
Characters being transmitted from the teletypewriter adapter to an OEM
device are always followed by two stop bits. Characters being received by
the teletypewriter adapter may have one or twostopbits. A start bit is
always a space or logical 0, and a stop bit is always a mark orlogical 1.
The transmit and receive lines are always held in the mark state when no
data is being transferred. Figure 4-4 shows the format of an 11-bit
transinitted or received character frame.
Because the teletypewriter adapter is a full-duplex attachment, where data
can be transmitted and received concurrently, OEM devices attached to
the teletypewriter adapter should be configured for full-duplex operation.
No error checking is done on transmitted or received data. The
teletypewriter adapter is code transparent. All 256 combinations of 8-bit
characters can be transmitted or received.
It is not possible to overrun on data being transmitted by the
teletypewriter adapter, but it is possible to overrun on data being received
by the teletypewriter adapter. If a second character is received before the
first character is serviced by the software, the first character is not lost,
but the second character is lost.
© Start bit
-— / pType ppg
t t '
! ' ' ' ' ' ' ' 1 St '
8 ' i i i ' ' op '
The line is always held in a mark condition when
no data is being transmitted.
The start bit is always a space (logical 0).
Eight data bits are transmitted for each character.
Each bit is either a mark or a space, depending on
the character code. Theleast-significant bit
is transmittedfirst.
The stop bit is always a mark (logical 1). On
transmit, two stop bits are always used; on receive,
one or two stop bits can be used.
Figure 4-4. Format of an 11-bit transmitted or received character frame
Teletypewriter Adapter Feature 411