Chapter 4 - Memory Expansion Module Installation
4-21
The following is some additional information on the usage of Control Reg-
ister 2.
Read only bits 7 and 6 indicate battery voltage. If the bit is a zero, the
battery is above the threshold (2.2 VDC). A one indicates the battery is
below the threshold (<2.2 Vdc) or not installed. Bit 7 is VB2, the bat-
tery connected to J5. Bit 6 is VB1, the battery connected to J4.
Bit 5 is the power fail alarm. A zero means system (
not battery
) voltage
has dropped below the voltage selected with W10.
Bit 4 is reserved. Always set this bit to zero when writing to register 2
(674h or 675h).
Bit 3 (-WEN0) provides a software controlled write protect/write enable
bit. It allows write protection of SRAMs and Flash EPROMs. If bit 3 is
zero, the PC bus signal -SMEMW is available to the byte-wide sockets,
allowing normal read/write operation. If bit 3 is a one, writing to de-
vices in the byte-wide sockets is inhibited. With writing inhibited, these
become read-only devices.
Bit 2 provides software control of the +12 Vpp for Flash EPROM pro-
gramming. Flash EPROM programming is discussed later in this chap-
ter. If this bit is one, +12 volts is routed to W2-3 and W7-3.
Bits 1 and 0 are used to select 64K byte pages of larger memory devices.
(See Figure 4-5.)
Reading the Registers
Both Control Registers are readable. Control Register 1 and most of 2 are
writable. Three read-only bits (7-5) in Control Register 2 provide informa-
tion about the state of the off board batteries and the power fail alarm.
Reading Control Register 2 is the same. Control Register 2 is located at
674h or 675h, but its output is controlled by the board number select logic.
To read Control Register 2 of Board Number 1 at address 674h, write to
register 1, at 274h, specifying Board Number 1. This will enable the output
of
only
Register 2 of that module to be read (at address 674h). To check the
status of the backup batteries on that module, read bits 6 and 7. The follow-
ing example is a simple assembly language routine that accesses these bits:
Table 4- 21. Assembly Language Routine Example
MOV
MOV
OUT
MOV
IN
AND
RET
AL, 40h
DX, 274h
DX, AL
DX, 674h
AL, DX
AL, 11oooooob
;
;
;
;
;
;
;
Board Number 1
Port Address 274h
Outputs Board Number to Control Port Address
Selects Control Register 2 as source of data
Inputs data from Control register 2
Masks all but Battery Monitor bits
Returns will not be Zero if a battery is low
Return
Summary of Contents for XVME-976/202
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