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data lines and 9 signaling lines. Additionally, the parallel port

passthrough function requires another set of the 9 control signals.

The LM9830 databus and control signals are tied to the PC’s par-

allel port. To support a parallel port passthrough function, the

LM9830’s control outputs are tri-stated to allow the printer to

communicate with the PC when in passthrough mode. When the

LM9830 is active, the printer is disabled by tri-stating all control

I/O between the printer and the LM9830/PC control bus. A more

detailed description of the parallel port passthrough function is

provided on the full page drawing labelled Printer Passthrough

Block Diagram near the end of this document.

To minimize EMI and on-chip noise, the Parallel Port output driv-

ers (D0-D7 and the 9 control/status output signals) have four out-

put current settings, 0-3. The output current level is set by bits 1

and 2 of Configuration Register 42.

For maximum compatibility and reliability, the “3” setting is recom-

mended. “0” - “2” can be used to reduce EMI and on-chip noise if

the final system (customer’s PC and associated peripherals and

cables) can tolerate it.

6.2 Finding the LM9830

The LM9830 powers up in the Transparent mode. In order to com-

municate with the LM9830, the host must send a specific

sequence of data on the databus without changing any of the 4

control signal lines. The LM9830 looks for the sequence 99 66

CC 33 on D0-D7.

Each state (99, 66, CC, and 33) must be held for a minimum of 4

MCLK cycles. After a power on reset status, the MCLK divider is

set to divide-by 4. This means that each state must be held for 16

CRYSTAL IN cycles. For a 50MHz external clock, this means that

each state must be held for a minimum of 16*20ns = 320ns. If the

MCLK divider is programmed to a different value and the LM9830

goes transparent, the minimum time required to wake up the

LM9830 will change. The equation for the length of time each

state must be held is:

t = 4(tCLK_IN)(MCLK_DIVIDER)

The assumption is that this sequence will not occur at random

without any of the 4 control pins violating their static requirement

(STROBE high, the other three static).

When in the transparent mode with a clock applied, the LM9830

constantly monitors the databus for a transition to 99. If 99 is

detected, the LM9830 looks for 66. If 66 is detected, the LM9830

looks for CC. If CC is detected, the LM9830 looks for 33.

If 33 is detected, the LM9830 exits transparent mode.

When the LM9830 exits the Transparent mode it takes the

TRISTATE pin high to disconnect the printer control signals to the

PC, and the LATCH signal low to latch and hold the current state

of the four control signals going to the printer. The 5 control lines

going back to the host change to their deasserted states:

ERROR = high

ACK = high

BUSY = low

PE = low

SELECT = low

At this point the LM9830 software driver can attempt to write to

and read from the configuration register to confirm the pres-

ence/non-presence of the LM9830. Please note that register 42

must be written correctly to allow the LM9830 to respond in the

desired communication mode (8 bit or nibble).

6.3 Selecting EPP or Nibble Mode I/O

Now that the LM9830 has been detected, the Host can start talk-

ing to it. The host PC always writes to the LM9830 using 8 bit

words. For reading data, the LM9830 can communicate in either

8 bit (Bidirectional or EPP) or 4 bit (Nibble) modes, as determined

by the state of register 42, bit 0. This bit has no power-on default

and must be set to a 0 or a 1 before data can be read from the

LM9830.

6.4 Returning to Transparency Mode Without LM9830 Reset

The host can return the LM9830 to Transparency Mode by taking

the INIT pin low and then high again. Approximately 2 - 3 MCLKs

after the rising edge of INIT, the LATCH pin will go high, the

TRISTATE pin will go low, and the LM9830 will tristate its D0-D7

and control line outputs. This will make the LM9830 transparent,

but will not change its operation state. If it was scanning, idling, or

fast feeding, the LM9830 will continue scanning, idling, or fast

Name

Direction

LM9830

Default

Parallel Port Databus

D0-D7

From (To) PC

TriState

PC Control Signals

STROBE

From PC

Input

AUTOFEED

From PC

Input

INIT

From PC

Input

SELECT IN

From PC

Input

ACK

To PC

High

BUSY

To PC

Low

To PC

Low

SELECT

To PC

Low

ERROR

To PC

High

Printer Passthrough Signals

TRISTATE

To External Buffer

Low

LATCH

To External Latch

Low

Figure 49: Printer Port Pin Description

Current

Setting

IOL

(mA)

IOH

(mA)

tF (ns)

200pF

tR (ns)

200pF

-6

200

167

-9

143

111

-12

111

-21

Figure 50: Printer Passthrough Overview

Buffer

Computer

Printer

LM9830

Databus

Control

Signals

Passthrough

Control

Signals

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