Inhibit (Pin 44)
The Inhibit output is asserted while data on the selected I/O port are being read into the I/O port buffer.
This line is normally active high but may be programmed active low (
I1
) by the
Invert
(
I
) command. The
Inhibit line can be programmed independent of any I/O operations with the
Inhibit
(
Q
) command.
The Inhibit line is asserted once for each data read operation for all
Format
(
F
) modes except high speed
binary (
F5
). In this mode, it is asserted for the first data read after the Digital488/80A is addressed to talk.
On the last data byte transfer, the data are read again with Inhibit asserted in anticipation of another data
transfer. If Inhibit is used to sequence external hardware, you should be aware that this line pulses
N+1
times where
N
is the number of total 5-byte data transfers.
The Inhibit Line is not pulsed when reading a particular bit status using the
User Status
(
U
) command.
Service (Pin 45)
The Service input is an edge sensitive input capable of generating a bus Service Request (
SRQ
). It is
enabled with the
Service Request Mask
(
M
) command and defaults to rising-edge sensitive. The
Invert
(
I
)
command can be used to program it to be falling-edge sensitive (
I64
).
External Data Ready (Pin 46)
The External Data Ready (EDR) line is an edge sensitive input used to latch input data. It is used in
conjunction with the
Data Ready
(
R
) command. The EDR signal must be at least 1 microsecond wide and
must have a rise and fall time of less than one microsecond. The EDR line is normally rising-edge
sensitive but can be programmed with the
Invert
(
I
) command to be falling-edge sensitive (
I32
).
When using the EDR line with the
Data Ready
(
R
) command
R1
, data is not read when the Digital488/80A
is addressed to talk, as it is read with command
R0
. The Digital488/80A only outputs data when the EDR
line transitions.
When using the EDR line with the
R2
command, data is read and stored in an internal data buffer each time
the EDR line transitions. The Digital488/80A only outputs data when there is data in the buffer to output.
EDR is not functional in the high speed binary (
F5
) format.
Digital488/80A User’s Manual
Digital488/80A Operation 31
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