DP8344 datasheet(47/184 Pages) NSC

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30 Transceiver (Continued)

The transceiver has several modes of operation It can be

configured for single line half-duplex operation in which the

receiver is disabled while the transmitter is active Alterna-

tively both receiver and transmitter can be active at the

same time for multi-channel (such as repeater) or loopback

operation The transceiver has both internal and external

loopback capabilities facilitating testing of both the soft-

ware and external hardware At all times both transmitter

and receiver operate according to the same protocol defini-

tion

311 Protocols

In all protocols data is transmitted serially in discrete mes-

sages containing one or more frames each representing a

single word of information Biphase (Manchester II) encod-

ing is used in which the data stream is divided into discrete

time intervals (bit-times) denoted by a level transition in the

center of the bit-time For the IBM 3270 3299 and NSC

general purpose 8-bit protocols a mid-bit transition from low

to high represents a biphase ‘‘1’’ and a mid-bit transition

from high to low represents a biphase ‘‘0’’ For the 5250

protocol the definition of biphase logic levels is exactly re-

versed ie a biphase ‘‘1’’ is represented by a high to low

transition Depending on the bit sequence there may or

may not be a transition on the bit-time boundary The bi-

phase encoding of a simple bit sequence is illustrated in

Figure 3-2(a)

Each transmission begins with a unique start sequence con-

sisting of 5 biphase encoded ‘‘1’s’’ (referred to as ‘‘line

quiesce pulses’’) followed by a 3 bit-time code violation and

the sync bit of the first frame

Figure 3-2(b) The three bit-

time code violation does not conform to the rules of Man-

chester encoding and forms a unique recognition pattern for

bit time synchronization by the receiver logic The first bit of

any frame is the sync bit a biphase ‘‘1’’ The frame is then

formatted according to the requirements of the protocol If a

multi-frame message is being transmitted additional frames

are appended to the end of the first frameexcept for the

5250 protocol where there may be an optional number of

‘‘fill bits’’ (biphase ‘‘0’’) between each frame

Depending on the protocol when all data has been trans-

mitted the end of a message will be indicated either by the

transmission of an ending sequence or (for 5250) simply by

the cessation of transitions on the differential line Later

model 5250 equipment has incorporated a ‘‘line hold’’ at the

end of the message The line hold maintains the final differ-

ential state on the line for several bit times to eliminate

noise or reflections that could be interpreted as a continu-

ance of the message The ending sequence for all but 5250

protocols consists of a single biphase ‘‘0’’ followed by a low

to high transition on the bit-time boundary and two bit-times

with no transitions (two mini-code violation)

Figure 3-2(c)

The various protocol framing formats are shown in

Figures

3-3 through 3-5 The diagrams use a bit pattern drawing

convention which for clarity shows the bit-time boundaries

but not the biphase transitions in the center of the bit times

The timing relationship between the biphase encoded bit

stream and the bit pattern diagrams is consistent with

Fig-

ure 3-2

TLF9336 – 34

(a) Biphase Encoding

TLF9336 – 36

(b) Starting Sequence

TLF9336 – 35

FIGURE 3-2 Biphase Encoding

3111 IBM 3270

The framing format of the IBM 3270 coax protocol is shown

Figures 3-3(a) and (b) for both single and multi-frame

messages Each message begins with a starting sequence

and ends with an ending sequence as shown in

Figures

3-2(b) and (c) Each 12-bit frame begins with a sync bit (B1)

followed by an 8-bit data byte (MSB first) a 2-bit control

field and the frame delimiter bit (B12) representing even

parity on the previous 11 bits The bit rate on the coax line is

23587 MHz

3112 IBM 3299

Adding 3299 multiplexers to the 3270 environment requires

an address to be transmitted along with each message from

the controller to the multiplexer The IBM 3299 Terminal

Multiplexer protocol provides this capability by defining an

additional 8-bit frame as the first frame of every message

sent from the controller as shown in

Figure 3-3(c) This

frame contains a 6-bit data field along with the normal sync

and word parity bits The protocol currently utilizes bits B2 –

B4 as an address field that directs the message through the

multiplexor hardware Following the address frame the rest

of the message follows standard 3270 convention The bit

rate 23587 MHz is the same as standard 3270

3113 IBM 5250

The framing format of the IBM 5250 twinax protocol is

shown in

Figure 3-4 for both single and multi-frame mes-

sages Each message begins with the starting sequence

shown in

Figure 3-2(b) and ends with 3 fill bits (biphase

‘‘0’’) A 16-bit frame is employed consisting of a sync bit

(B15) an 8-bit data byte (B7 – B14) (LSB first) a 3-bit station

address field (B4 – B6) and the last bit (B3) representing

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